Fixes Grammar in SourceTextContainer exception message

src/EditorFeatures/Test/EditorServicesTest.csproj

@@ -242,6 +242,7 @@
     <Compile Include="Diagnostics\GenerateType\TestProjectManagementService.cs" />
     <Compile Include="Diagnostics\SuppressMessageAttributeTests.cs" />
     <Compile Include="CodeFixes\ExtensionOrderingTests.cs" />
+    <Compile Include="Extensions\SourceTextContainerExtensionsTests.cs" />
     <Compile Include="Preview\TestOnly_CompilerDiagnosticAnalyzerProviderService.cs" />
     <Compile Include="DocCommentFormatting\DocCommentFormattingTests.cs" />
     <Compile Include="DocumentationComments\AbstractDocumentationCommentTests.cs" />

src/EditorFeatures/Test/Extensions/SourceTextContainerExtensionsTests.cs

@@ -0,0 +1,34 @@
+using Microsoft.CodeAnalysis.Text;
+using Moq;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.Editor.UnitTests.Extensions
+{
+    public class SourceTextContainerExtensionsTests
+    {
+        [Fact]
+        public void GetBufferTextFromNonTextContainerThrows()
+        {
+            var containerMock = new Mock<SourceTextContainer>();
+            Assert.Throws<ArgumentException>(() => Microsoft.CodeAnalysis.Text.Extensions.GetTextBuffer(containerMock.Object));
+        }
+
+        [Fact]
+        public void GetBufferTextFromTextContainerDoesNotThrow()
+        {
+            var textSnapshotMock = new Mock<VisualStudio.Text.ITextSnapshot>();
+            var bufferMock = new Mock<VisualStudio.Text.ITextBuffer>();
+            bufferMock.SetupGet(x => x.CurrentSnapshot).Returns(textSnapshotMock.Object);
+            bufferMock.SetupGet(x => x.Properties).Returns(new VisualStudio.Utilities.PropertyCollection());
+
+            var textContainer = Microsoft.CodeAnalysis.Text.Extensions.TextBufferContainer.From(bufferMock.Object);
+
+            Assert.DoesNotThrow(()=> Microsoft.CodeAnalysis.Text.Extensions.GetTextBuffer(textContainer));
+        }
+    }
+}

src/EditorFeatures/Text/Extensions.cs

@@ -25,7 +25,7 @@ public static ITextBuffer GetTextBuffer(this SourceTextContainer textContainer)
             var textBuffer = TryGetTextBuffer(textContainer);
             if (textBuffer == null)
             {
-                throw new ArgumentException(TextEditorResources.TextcontainerIsNotAnSource, "textContainer");
+                throw new ArgumentException(TextEditorResources.TextContainerNotFromTextBuffer, nameof(textContainer));
             }
 
             return textBuffer;

src/EditorFeatures/Text/TextEditorResources.resx

@@ -117,10 +117,7 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
-  <data name="TextcontainerIsNotAnSource" xml:space="preserve">
-    <value>textContainer is not an SourceTextContainer that was created from an ITextBuffer.</value>
-  </data>
-  <data name="TextIsNotAnSourcetextThat" xml:space="preserve">
-    <value>text is not an SourceText that was created from an ITextSnapshot.</value>
+  <data name="TextContainerNotFromTextBuffer" xml:space="preserve">
+    <value>textContainer is not a SourceTextContainer that was created from an ITextBuffer.</value>
   </data>
 </root>