Add analyzer to warn for invalid Component parameter usage.

- The newly added analyzer warns users if they try to assign another components parameter.It does sanity checks to ensure that
  1. The property reference is indeed a component parameter
  2. The property reference is from a component
  3. The assignment is outside of the parameters type hierarchy. Aka, we don't warn users for setting a components parameter if it's in the same class.

- Updated existing `ComponentsFacts` to add additional utility methods to properly interact with components.
- Added tests to ensure we're analyzing all the goods properly.

#8825

src/Components/Analyzers/src/ComponentFacts.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Linq;
 using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
 
 namespace Microsoft.AspNetCore.Components.Analyzers
 {
@@ -85,5 +86,26 @@ namespace Microsoft.AspNetCore.Components.Analyzers
 
             return property.GetAttributes().Any(a => a.AttributeClass == symbols.CascadingParameterAttribute);
         }
+
+        public static bool IsComponent(ComponentSymbols symbols, Compilation compilation, INamedTypeSymbol type)
+        {
+            if (symbols is null)
+            {
+                throw new ArgumentNullException(nameof(symbols));
+            }
+
+            if (type is null)
+            {
+                throw new ArgumentNullException(nameof(type));
+            }
+
+            var conversion = compilation.ClassifyConversion(symbols.IComponentType, type);
+            if (!conversion.Exists || !conversion.IsExplicit)
+            {
+                return false;
+            }
+
+            return true;
+        }
     }
 }

src/Components/Analyzers/src/ComponentParameterUsageAnalyzer.cs

@@ -0,0 +1,104 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Immutable;
+using System.Linq;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Operations;
+
+namespace Microsoft.AspNetCore.Components.Analyzers
+{
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    public class ComponentParameterUsageAnalyzer : DiagnosticAnalyzer
+    {
+        public ComponentParameterUsageAnalyzer()
+        {
+            SupportedDiagnostics = ImmutableArray.Create(new[]
+            {
+                DiagnosticDescriptors.ComponentParametersShouldNotBeSetOutsideOfTheirDeclaredComponent,
+            });
+        }
+
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; }
+
+        public override void Initialize(AnalysisContext context)
+        {
+            context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.Analyze | GeneratedCodeAnalysisFlags.ReportDiagnostics);
+            context.RegisterCompilationStartAction(context =>
+            {
+                if (!ComponentSymbols.TryCreate(context.Compilation, out var symbols))
+                {
+                    // Types we need are not defined.
+                    return;
+                }
+
+                context.RegisterOperationBlockStartAction(startBlockContext =>
+                {
+                    startBlockContext.RegisterOperationAction(context =>
+                    {
+                        var assignmentOperation = (IAssignmentOperation)context.Operation;
+                        var leftHandSide = assignmentOperation.Target;
+                        if (leftHandSide == null)
+                        {
+                            // Malformed assignment, no left hand side.
+                            return;
+                        }
+
+                        if (leftHandSide.Kind != OperationKind.PropertyReference)
+                        {
+                            // We don't want to capture situations where a user does
+                            // MyOtherProperty = aComponent.SomeParameter
+                            return;
+                        }
+
+                        var propertyReference = (IPropertyReferenceOperation)leftHandSide;
+                        var componentProperty = (IPropertySymbol)propertyReference.Member;
+
+                        if (!ComponentFacts.IsParameter(symbols, componentProperty))
+                        {
+                            // This is not a property reference that we care about, it is not decorated with [Parameter].
+                            return;
+                        }
+
+                        var propertyContainingType = componentProperty.ContainingType;
+                        if (!ComponentFacts.IsComponent(symbols, context.Compilation, propertyContainingType))
+                        {
+                            // Someone referenced a property as [Parameter] inside something that is not a component.
+                            return;
+                        }
+
+                        var assignmentContainingType = startBlockContext.OwningSymbol?.ContainingType;
+                        if (assignmentContainingType == null)
+                        {
+                            // Assignment location has no containing type. Most likely we're operating on malformed code, don't try and validate.
+                            return;
+                        }
+
+                        var conversion = context.Compilation.ClassifyConversion(propertyContainingType, assignmentContainingType);
+                        if (conversion.Exists && conversion.IsIdentity)
+                        {
+                            // The assignment is taking place inside of the declaring component.
+                            return;
+                        }
+
+                        if (conversion.Exists && conversion.IsExplicit)
+                        {
+                            // The assignment is taking place within the components type hierarchy. This means the user is setting this in a supported
+                            // scenario.
+                            return;
+                        }
+
+                        // At this point the user is referencing a component parameter outside of its declaring class.
+
+                        context.ReportDiagnostic(Diagnostic.Create(
+                            DiagnosticDescriptors.ComponentParametersShouldNotBeSetOutsideOfTheirDeclaredComponent,
+                            propertyReference.Syntax.GetLocation(),
+                            propertyReference.Member.Name));
+                    }, OperationKind.SimpleAssignment, OperationKind.CompoundAssignment, OperationKind.CoalesceAssignment);
+                });
+            });
+        }
+    }
+}

src/Components/Analyzers/src/ComponentSymbols.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Collections.Generic;
 using Microsoft.CodeAnalysis;
 
 namespace Microsoft.AspNetCore.Components.Analyzers
@@ -30,6 +29,13 @@ namespace Microsoft.AspNetCore.Components.Analyzers
                 return false;
             }
 
+            var icomponentType = compilation.GetTypeByMetadataName(ComponentsApi.IComponent.MetadataName);
+            if (icomponentType == null)
+            {
+                symbols = null;
+                return false;
+            }
+
             var dictionary = compilation.GetTypeByMetadataName("System.Collections.Generic.Dictionary`2");
             var @string = compilation.GetSpecialType(SpecialType.System_String);
             var @object = compilation.GetSpecialType(SpecialType.System_Object);
@@ -41,18 +47,24 @@ namespace Microsoft.AspNetCore.Components.Analyzers
 
             var parameterCaptureUnmatchedValuesRuntimeType = dictionary.Construct(@string, @object);
 
-            symbols = new ComponentSymbols(parameterAttribute, cascadingParameterAttribute, parameterCaptureUnmatchedValuesRuntimeType);
+            symbols = new ComponentSymbols(
+                parameterAttribute,
+                cascadingParameterAttribute,
+                parameterCaptureUnmatchedValuesRuntimeType,
+                icomponentType);
             return true;
         }
 
         private ComponentSymbols(
             INamedTypeSymbol parameterAttribute,
             INamedTypeSymbol cascadingParameterAttribute,
-            INamedTypeSymbol parameterCaptureUnmatchedValuesRuntimeType)
+            INamedTypeSymbol parameterCaptureUnmatchedValuesRuntimeType,
+            INamedTypeSymbol icomponentType)
         {
             ParameterAttribute = parameterAttribute;
             CascadingParameterAttribute = cascadingParameterAttribute;
             ParameterCaptureUnmatchedValuesRuntimeType = parameterCaptureUnmatchedValuesRuntimeType;
+            IComponentType = icomponentType;
         }
 
         public INamedTypeSymbol ParameterAttribute { get; }
@@ -61,5 +73,7 @@ namespace Microsoft.AspNetCore.Components.Analyzers
         public INamedTypeSymbol ParameterCaptureUnmatchedValuesRuntimeType { get; }
 
         public INamedTypeSymbol CascadingParameterAttribute { get; }
+
+        public INamedTypeSymbol IComponentType { get; }
     }
 }

src/Components/Analyzers/src/ComponentsApi.cs

@@ -22,5 +22,11 @@ namespace Microsoft.AspNetCore.Components.Analyzers
             public static readonly string FullTypeName = "Microsoft.AspNetCore.Components.CascadingParameterAttribute";
             public static readonly string MetadataName = FullTypeName;
         }
+
+        public static class IComponent
+        {
+            public static readonly string FullTypeName = "Microsoft.AspNetCore.Components.IComponent";
+            public static readonly string MetadataName = FullTypeName;
+        }
     }
 }

src/Components/Analyzers/src/DiagnosticDescriptors.cs

@@ -46,5 +46,14 @@ namespace Microsoft.AspNetCore.Components.Analyzers
             DiagnosticSeverity.Warning,
             isEnabledByDefault: true,
             description: new LocalizableResourceString(nameof(Resources.ComponentParametersShouldBePublic_Description), Resources.ResourceManager, typeof(Resources)));
+
+        public static readonly DiagnosticDescriptor ComponentParametersShouldNotBeSetOutsideOfTheirDeclaredComponent = new DiagnosticDescriptor(
+            "BL0005",
+            new LocalizableResourceString(nameof(Resources.ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Title), Resources.ResourceManager, typeof(Resources)),
+            new LocalizableResourceString(nameof(Resources.ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Format), Resources.ResourceManager, typeof(Resources)),
+            "Usage",
+            DiagnosticSeverity.Warning,
+            isEnabledByDefault: true,
+            description: new LocalizableResourceString(nameof(Resources.ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Description), Resources.ResourceManager, typeof(Resources)));
     }
 }

src/Components/Analyzers/src/Resources.Designer.cs

@@ -159,6 +159,33 @@ namespace Microsoft.AspNetCore.Components.Analyzers {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Component parameters should not be set outside of their declared component. Doing so may result in unexpected behavior at runtime..
+        /// </summary>
+        internal static string ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Description {
+            get {
+                return ResourceManager.GetString("ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Description", resourceCulture);
+            }
+        }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to Component parameter &apos;{0}&apos; should not be set outside of its component..
+        /// </summary>
+        internal static string ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Format {
+            get {
+                return ResourceManager.GetString("ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Format", resourceCulture);
+            }
+        }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to Component parameter should not be set outside of its component..
+        /// </summary>
+        internal static string ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Title {
+            get {
+                return ResourceManager.GetString("ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Title", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Component parameters should be public..
         /// </summary>

src/Components/Analyzers/src/Resources.resx

@@ -156,4 +156,13 @@
   <data name="ComponentParametersShouldBePublic_FixTitle" xml:space="preserve">
     <value>Make component parameters public.</value>
   </data>
+  <data name="ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Description" xml:space="preserve">
+    <value>Component parameters should not be set outside of their declared component. Doing so may result in unexpected behavior at runtime.</value>
+  </data>
+  <data name="ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Format" xml:space="preserve">
+    <value>Component parameter '{0}' should not be set outside of its component.</value>
+  </data>
+  <data name="ComponentParameterShouldNotBeSetOutsideOfTheirDeclaredComponent_Title" xml:space="preserve">
+    <value>Component parameter should not be set outside of its component.</value>
+  </data>
 </root>

src/Components/Analyzers/test/ComponentParameterUsageAnalyzerTest.cs

@@ -0,0 +1,298 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+using TestHelper;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Components.Analyzers
+{
+    public class ComponentParameterUsageAnalyzerTest : DiagnosticVerifier
+    {
+        public ComponentParameterUsageAnalyzerTest()
+        {
+            ComponentTestSource = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class TestComponent : IComponent
+        {{
+            [Parameter] public string TestProperty {{ get; set; }}
+            public string NonParameter {{ get; set; }}
+        }}
+    }}" + ComponentsTestDeclarations.Source;
+        }
+
+        private string ComponentTestSource { get; }
+
+        [Fact]
+        public void ComponentPropertySimpleAssignment_Warns()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class OtherComponent : IComponent
+        {{
+            private TestComponent _testComponent;
+            void Render()
+            {{
+                _testComponent = new TestComponent();
+                _testComponent.TestProperty = ""Hello World"";
+            }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test,
+                new DiagnosticResult
+                {
+                    Id = DiagnosticDescriptors.ComponentParametersShouldNotBeSetOutsideOfTheirDeclaredComponent.Id,
+                    Message = "Component parameter 'TestProperty' should not be set outside of its component.",
+                    Severity = DiagnosticSeverity.Warning,
+                    Locations = new[]
+                    {
+                        new DiagnosticResultLocation("Test0.cs", 11, 17)
+                    }
+                });
+        }
+
+        [Fact]
+        public void ComponentPropertyCoalesceAssignment__Warns()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class OtherComponent : IComponent
+        {{
+            private TestComponent _testComponent;
+            void Render()
+            {{
+                _testComponent = new TestComponent();
+                _testComponent.TestProperty ??= ""Hello World"";
+            }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test,
+                new DiagnosticResult
+                {
+                    Id = DiagnosticDescriptors.ComponentParametersShouldNotBeSetOutsideOfTheirDeclaredComponent.Id,
+                    Message = "Component parameter 'TestProperty' should not be set outside of its component.",
+                    Severity = DiagnosticSeverity.Warning,
+                    Locations = new[]
+                    {
+                        new DiagnosticResultLocation("Test0.cs", 11, 17)
+                    }
+                });
+        }
+
+        [Fact]
+        public void ComponentPropertyCompoundAssignment__Warns()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class OtherComponent : IComponent
+        {{
+            private TestComponent _testComponent;
+            void Render()
+            {{
+                _testComponent = new TestComponent();
+                _testComponent.TestProperty += ""Hello World"";
+            }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test,
+                new DiagnosticResult
+                {
+                    Id = DiagnosticDescriptors.ComponentParametersShouldNotBeSetOutsideOfTheirDeclaredComponent.Id,
+                    Message = "Component parameter 'TestProperty' should not be set outside of its component.",
+                    Severity = DiagnosticSeverity.Warning,
+                    Locations = new[]
+                    {
+                        new DiagnosticResultLocation("Test0.cs", 11, 17)
+                    }
+                });
+        }
+
+        [Fact]
+        public void ComponentPropertyExpression_Ignores()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class TypeName
+        {{
+            void Method()
+            {{
+                System.IO.Console.WriteLine(new TestComponent().TestProperty);
+            }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test);
+        }
+
+        [Fact]
+        public void ComponentPropertyExpressionInStatement_Ignores()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class TypeName
+        {{
+            void Method()
+            {{
+                var testComponent = new TestComponent();
+                for (var i = 0; i < testComponent.TestProperty.Length; i++)
+                {{
+                }}
+            }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test);
+        }
+
+        [Fact]
+        public void RetrievalOfComponentPropertyValueInAssignment_Ignores()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class TypeName
+        {{
+            void Method()
+            {{
+                var testComponent = new TestComponent();
+                AnotherProperty = testComponent.TestProperty;
+            }}
+
+            public string AnotherProperty {{ get; set; }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test);
+        }
+
+        [Fact]
+        public void ShadowedComponentPropertyAssignment_Ignores()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class TypeName
+        {{
+            void Method()
+            {{
+                var testComponent = new InheritedComponent();
+                testComponent.TestProperty = ""Hello World"";
+            }}
+        }}
+
+        class InheritedComponent : TestComponent
+        {{
+            public new string TestProperty {{ get; set; }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test);
+        }
+
+        [Fact]
+        public void InheritedImplicitComponentPropertyAssignment_Ignores()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class TypeName : TestComponent
+        {{
+            void Method()
+            {{
+                this.TestProperty = ""Hello World"";
+            }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test);
+        }
+
+        [Fact]
+        public void ImplicitComponentPropertyAssignment_Ignores()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class TypeName : IComponent
+        {{
+            void Method()
+            {{
+                TestProperty = ""Hello World"";
+            }}
+
+            [Parameter] public string TestProperty {{ get; set; }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test);
+        }
+
+        [Fact]
+        public void ComponentPropertyAssignment_NonParameter_Ignores()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class OtherComponent : IComponent
+        {{
+            private TestComponent _testComponent;
+            void Render()
+            {{
+                _testComponent = new TestComponent();
+                _testComponent.NonParameter = ""Hello World"";
+            }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test);
+        }
+
+        [Fact]
+        public void NonComponentPropertyAssignment_Ignores()
+        {
+            var test = $@"
+    namespace ConsoleApplication1
+    {{
+        using {typeof(ParameterAttribute).Namespace};
+        class OtherComponent : IComponent
+        {{
+            private SomethingElse _testNonComponent;
+            void Render()
+            {{
+                _testNonComponent = new NotAComponent();
+                _testNonComponent.TestProperty = ""Hello World"";
+            }}
+        }}
+        class NotAComponent
+        {{
+            [Parameter] public string TestProperty {{ get; set; }}
+        }}
+    }}" + ComponentTestSource;
+
+            VerifyCSharpDiagnostic(test);
+        }
+
+        protected override DiagnosticAnalyzer GetCSharpDiagnosticAnalyzer() => new ComponentParameterUsageAnalyzer();
+    }
+}

src/Components/Analyzers/test/ComponentsTestDeclarations.cs

@@ -16,6 +16,10 @@ namespace Microsoft.AspNetCore.Components.Analyzers
         public class {typeof(CascadingParameterAttribute).Name} : System.Attribute
         {{
         }}
+
+        public interface {typeof(IComponent).Name}
+        {{
+        }}
     }}
 ";
     }