Add ParameterCollection.GetValueOrDefault methods

src/Microsoft.AspNetCore.Blazor/Components/ParameterCollection.cs

@@ -64,6 +64,27 @@ namespace Microsoft.AspNetCore.Blazor.Components
             return false;
         }
 
+        /// <summary>
+        /// Gets the value of the parameter with the specified name, or a default value
+        /// if no such parameter exists in the collection.
+        /// </summary>
+        /// <typeparam name="T">The type of the value.</typeparam>
+        /// <param name="parameterName">The name of the parameter.</param>
+        /// <returns>The parameter value if found; otherwise the default value for the specified type.</returns>
+        public T GetValueOrDefault<T>(string parameterName)
+            => GetValueOrDefault<T>(parameterName, default);
+
+        /// <summary>
+        /// Gets the value of the parameter with the specified name, or a specified default value
+        /// if no such parameter exists in the collection.
+        /// </summary>
+        /// <typeparam name="T">The type of the value.</typeparam>
+        /// <param name="parameterName">The name of the parameter.</param>
+        /// <param name="defaultValue">The default value to return if no such parameter exists in the collection.</param>
+        /// <returns>The parameter value if found; otherwise <paramref name="defaultValue"/>.</returns>
+        public T GetValueOrDefault<T>(string parameterName, T defaultValue)
+            => TryGetValue<T>(parameterName, out T result) ? result : defaultValue;
+
         /// <summary>
         /// Returns a dictionary populated with the contents of the <see cref="ParameterCollection"/>.
         /// </summary>

test/Microsoft.AspNetCore.Blazor.Test/ParameterCollectionTest.cs

@@ -121,6 +121,60 @@ namespace Microsoft.AspNetCore.Blazor.Test
             Assert.Equal("hello", value);
         }
 
+        [Fact]
+        public void CanGetValueOrDefault_WithExistingValue()
+        {
+            // Arrange
+            var myEntryValue = new object();
+            var parameterCollection = new ParameterCollection(new[]
+            {
+                RenderTreeFrame.Element(0, "some element").WithElementSubtreeLength(2),
+                RenderTreeFrame.Attribute(1, "my entry", myEntryValue),
+                RenderTreeFrame.Attribute(1, "my other entry", new object())
+            }, 0);
+
+            // Act
+            var result = parameterCollection.GetValueOrDefault<object>("my entry");
+
+            // Assert
+            Assert.Same(myEntryValue, result);
+        }
+
+        [Fact]
+        public void CanGetValueOrDefault_WithNonExistingValue()
+        {
+            // Arrange
+            var parameterCollection = new ParameterCollection(new[]
+            {
+                RenderTreeFrame.Element(0, "some element").WithElementSubtreeLength(2),
+                RenderTreeFrame.Attribute(1, "some other entry", new object())
+            }, 0);
+
+            // Act
+            var result = parameterCollection.GetValueOrDefault<DateTime>("nonexisting entry");
+
+            // Assert
+            Assert.Equal(default, result);
+        }
+
+        [Fact]
+        public void CanGetValueOrDefault_WithNonExistingValueAndExplicitDefault()
+        {
+            // Arrange
+            var explicitDefaultValue = new DateTime(2018, 3, 20);
+            var parameterCollection = new ParameterCollection(new[]
+            {
+                RenderTreeFrame.Element(0, "some element").WithElementSubtreeLength(2),
+                RenderTreeFrame.Attribute(1, "some other entry", new object())
+            }, 0);
+
+            // Act
+            var result = parameterCollection.GetValueOrDefault("nonexisting entry", explicitDefaultValue);
+
+            // Assert
+            Assert.Equal(explicitDefaultValue, result);
+        }
+
         [Fact]
         public void ThrowsIfTryGetExistingValueWithIncorrectType()
         {