Avoid a few allocations in StringContent

src/libraries/System.Net.Http/src/System/Net/Http/StringContent.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
 using System.IO;
 using System.Net.Http.Headers;
 using System.Text;
@@ -45,8 +46,31 @@ public StringContent(string content, Encoding? encoding)
         /// <param name="encoding">The encoding to use for the content.</param>
         /// <param name="mediaType">The media type to use for the content.</param>
         public StringContent(string content, Encoding? encoding, string mediaType)
-            : this(content, encoding, new MediaTypeHeaderValue(mediaType ?? DefaultMediaType, (encoding ?? DefaultStringEncoding).WebName))
+            : base(GetContentByteArray(content, encoding))
         {
+            Debug.Assert(DefaultStringEncoding.WebName == "utf-8");
+
+            encoding ??= DefaultStringEncoding;
+            mediaType ??= DefaultMediaType;
+
+            // Avoid allocating MediaTypeHeaderValue and related objects for common media types.
+            if (ReferenceEquals(encoding, DefaultStringEncoding))
+            {
+                string? knownValue = mediaType switch
+                {
+                    "text/plain" => "text/plain; charset=utf-8",
+                    "application/json" => "application/json; charset=utf-8",
+                    _ => null
+                };
+
+                if (knownValue is not null)
+                {
+                    Headers.TryAddWithoutValidation(KnownHeaders.ContentType.Descriptor, knownValue);
+                    return;
+                }
+            }
+
+            Headers.ContentType = new MediaTypeHeaderValue(mediaType, encoding.WebName);
         }
 
         /// <summary>Creates a new instance of the <see cref="StringContent"/> class.</summary>

src/libraries/System.Net.Http/tests/UnitTests/StringContentTests.cs

@@ -0,0 +1,66 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Net.Http.Headers;
+using System.Text;
+using Xunit;
+
+namespace System.Net.Http.Tests
+{
+    public class StringContentTests
+    {
+        [Fact]
+        public void StringContent_SetsDefaultContentTypeHeader()
+        {
+            var content = new StringContent("Foo");
+            Assert.Equal("text/plain; charset=utf-8", content.Headers.ContentType.ToString());
+
+            content = new StringContent("Foo", (Encoding)null);
+            Assert.Equal("text/plain; charset=utf-8", content.Headers.ContentType.ToString());
+
+            content = new StringContent("Foo", Encoding.UTF8);
+            Assert.Equal("text/plain; charset=utf-8", content.Headers.ContentType.ToString());
+
+            content = new StringContent("Foo", Encoding.UTF8, "text/plain");
+            Assert.Equal("text/plain; charset=utf-8", content.Headers.ContentType.ToString());
+
+            content = new StringContent("Foo", null, "text/plain");
+            Assert.Equal("text/plain; charset=utf-8", content.Headers.ContentType.ToString());
+
+            content = new StringContent("Foo", Encoding.UTF8, (string)null);
+            Assert.Equal("text/plain; charset=utf-8", content.Headers.ContentType.ToString());
+
+            content = new StringContent("Foo", null, (string)null);
+            Assert.Equal("text/plain; charset=utf-8", content.Headers.ContentType.ToString());
+
+            content = new StringContent("Foo", (MediaTypeHeaderValue)null);
+            Assert.Null(content.Headers.ContentType);
+
+            content = new StringContent("Foo", null, (MediaTypeHeaderValue)null);
+            Assert.Null(content.Headers.ContentType);
+        }
+
+        [Theory]
+        [InlineData("text/plain")]
+        [InlineData("application/json")]
+        [InlineData("application/xml")]
+        [InlineData("foo/bar")]
+        public void StringContent_SetsContentTypeHeader(string mediaType)
+        {
+            var content = new StringContent("foo", Encoding.UTF8, mediaType);
+            Assert.Equal($"{mediaType}; charset=utf-8", content.Headers.ContentType.ToString());
+
+            content = new StringContent("foo", null, mediaType);
+            Assert.Equal($"{mediaType}; charset=utf-8", content.Headers.ContentType.ToString());
+
+            content = new StringContent("foo", Encoding.ASCII, mediaType);
+            Assert.Equal($"{mediaType}; charset=us-ascii", content.Headers.ContentType.ToString());
+
+            content = new StringContent("foo", new MediaTypeHeaderValue(mediaType));
+            Assert.Equal(mediaType, content.Headers.ContentType.ToString());
+
+            content = new StringContent("foo", Encoding.UTF8, new MediaTypeHeaderValue(mediaType, "ascii"));
+            Assert.Equal($"{mediaType}; charset=ascii", content.Headers.ContentType.ToString());
+        }
+    }
+}

src/libraries/System.Net.Http/tests/UnitTests/System.Net.Http.Unit.Tests.csproj

@@ -329,6 +329,7 @@
     <Compile Include="MockContent.cs" />
     <Compile Include="RuntimeSettingParserTest.cs" />
     <Compile Include="StreamToStreamCopyTest.cs" />
+    <Compile Include="StringContentTests.cs" />
     <Compile Include="HttpEnvironmentProxyTest.cs" />
     <Compile Include="HttpRequestExceptionTests.cs" />
     <Compile Include="HttpWindowsProxyTest.cs" />