Modified exception message for duplicate keys exception in CandidateResolver

Addresses #5289

src/Microsoft.AspNetCore.Mvc.Core/Internal/DefaultAssemblyPartDiscoveryProvider.cs

@@ -7,6 +7,7 @@ using System.Diagnostics;
 using System.Linq;
 using System.Reflection;
 using Microsoft.AspNetCore.Mvc.ApplicationParts;
+using Microsoft.AspNetCore.Mvc.Core;
 using Microsoft.Extensions.DependencyModel;
 
 namespace Microsoft.AspNetCore.Mvc.Internal
@@ -73,8 +74,17 @@ namespace Microsoft.AspNetCore.Mvc.Internal
 
             public CandidateResolver(IReadOnlyList<RuntimeLibrary> dependencies, ISet<string> referenceAssemblies)
             {
-                _dependencies = dependencies
+                var dependenciesWithNoDuplicates = new Dictionary<string, Dependency>(StringComparer.OrdinalIgnoreCase);
-                    .ToDictionary(d => d.Name, d => CreateDependency(d, referenceAssemblies), StringComparer.OrdinalIgnoreCase);
+                foreach (var dependency in dependencies)
+                {
+                    if (dependenciesWithNoDuplicates.ContainsKey(dependency.Name))
+                    {
+                        throw new InvalidOperationException(Resources.FormatCandidateResolver_DifferentCasedReference(dependency.Name));
+                    }
+                    dependenciesWithNoDuplicates.Add(dependency.Name, CreateDependency(dependency, referenceAssemblies));
+                }
+
+                _dependencies = dependenciesWithNoDuplicates;
             }
 
             private Dependency CreateDependency(RuntimeLibrary library, ISet<string> referenceAssemblies)

src/Microsoft.AspNetCore.Mvc.Core/Properties/Resources.Designer.cs

@@ -1354,6 +1354,22 @@ namespace Microsoft.AspNetCore.Mvc.Core
             return string.Format(CultureInfo.CurrentCulture, GetString("VaryByQueryKeys_Requires_ResponseCachingMiddleware"), p0);
         }
 
+        /// <summary>
+        /// A duplicate entry for library reference {0} was found. Please check that all package references in all projects use the same casing for the same package references.
+        /// </summary>
+        internal static string CandidateResolver_DifferentCasedReference
+        {
+            get { return GetString("CandidateResolver_DifferentCasedReference"); }
+        }
+
+        /// <summary>
+        /// A duplicate entry for library reference {0} was found. Please check that all package references in all projects use the same casing for the same package references.
+        /// </summary>
+        internal static string FormatCandidateResolver_DifferentCasedReference(object p0)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("CandidateResolver_DifferentCasedReference"), p0);
+        }
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNetCore.Mvc.Core/Resources.resx

@@ -379,4 +379,8 @@
   <data name="VaryByQueryKeys_Requires_ResponseCachingMiddleware" xml:space="preserve">
     <value>'{0}' requires the response cache middleware.</value>
   </data>
+  <data name="CandidateResolver_DifferentCasedReference" xml:space="preserve">
+    <value>A duplicate entry for library reference {0} was found. Please check that all package references in all projects use the same casing for the same package references.</value>
+    <comment>{0} is the dependency name.</comment>
+  </data>
 </root>

test/Microsoft.AspNetCore.Mvc.Core.Test/Internal/DefaultAssemblyPartDiscoveryProviderTest.cs

@@ -14,6 +14,31 @@ namespace Microsoft.AspNetCore.Mvc.Internal
         private static readonly Assembly CurrentAssembly =
             typeof(DefaultAssemblyPartDiscoveryProviderTests).GetTypeInfo().Assembly;
 
+        [Fact]
+        public void CandidateResolver_ThrowsIfDependencyContextContainsDuplicateRuntimeLibraryNames()
+        {
+            // Arrange 
+            var upperCaseLibrary = "Microsoft.AspNetCore.Mvc";
+            var mixedCaseLibrary = "microsoft.aspNetCore.mvc";
+
+            var dependencyContext = new DependencyContext(
+                new TargetInfo("framework", "runtime", "signature", isPortable: true),
+                CompilationOptions.Default,
+                new CompilationLibrary[0],
+                new[]
+                {
+                     GetLibrary(mixedCaseLibrary),
+                     GetLibrary(upperCaseLibrary),
+                },
+                Enumerable.Empty<RuntimeFallbacks>());
+
+            // Act
+            var exception = Assert.Throws<InvalidOperationException>(() => DefaultAssemblyPartDiscoveryProvider.GetCandidateLibraries(dependencyContext));
+
+            // Assert
+            Assert.Equal($"A duplicate entry for library reference {upperCaseLibrary} was found. Please check that all package references in all projects use the same casing for the same package references.", exception.Message);
+        }
+
         [Fact]
         public void GetCandidateLibraries_IgnoresMvcAssemblies()
         {