Better heuristics for extension methods returning IDisposable. Fix #96.

IDisposableAnalyzers.Test/Helpers/DisposableTests.IsCreation.cs

@@ -503,6 +503,54 @@ public void Dispose()
                 var value = syntaxTree.FindInvocation("disposable.AddAndReturn(File.OpenRead(string.Empty))");
                 Assert.AreEqual(Result.No, Disposable.IsCreation(value, semanticModel, CancellationToken.None));
             }
+
+            [Test]
+            public void AssumeYesForExtensionMethodReturningSameTypeAsThisParameter()
+            {
+                var testCode = @"
+namespace RoslynSandbox
+{
+    using System;
+    using Stubs;
+
+    class Foo
+    {
+        public Foo(IDisposable disposable)
+        {
+            var custom = disposable.AsCustom();
+        }
+    }
+}";
+                var syntaxTree = CSharpSyntaxTree.ParseText(testCode);
+                var compilation = CSharpCompilation.Create("test", new[] { syntaxTree }, MetadataReferences.FromAttributes());
+                var semanticModel = compilation.GetSemanticModel(syntaxTree);
+                var value = syntaxTree.FindInvocation("disposable.AsCustom()");
+                Assert.AreEqual(Result.AssumeYes, Disposable.IsCreation(value, semanticModel, CancellationToken.None));
+            }
+
+            [Test]
+            public void AssumeNoForExtensionMethodReturningSameTypeAsThisParameter()
+            {
+                var testCode = @"
+namespace RoslynSandbox
+{
+    using System;
+    using Stubs;
+
+    class Foo
+    {
+        public Foo(IDisposable disposable)
+        {
+            var custom = disposable.Fluent();
+        }
+    }
+}";
+                var syntaxTree = CSharpSyntaxTree.ParseText(testCode);
+                var compilation = CSharpCompilation.Create("test", new[] { syntaxTree }, MetadataReferences.FromAttributes());
+                var semanticModel = compilation.GetSemanticModel(syntaxTree);
+                var value = syntaxTree.FindInvocation("disposable.Fluent()");
+                Assert.AreEqual(Result.AssumeNo, Disposable.IsCreation(value, semanticModel, CancellationToken.None));
+            }
         }
     }
 }

IDisposableAnalyzers.Test/IDisposableAnalyzers.Test.csproj

@@ -27,6 +27,9 @@
   <ItemGroup>
     <Reference Include="PresentationCore" />
     <Reference Include="PresentationFramework" />
+    <Reference Include="Stubs">
+      <HintPath>Lib\Stubs.dll</HintPath>
+    </Reference>
     <Reference Include="System.Xaml" />
     <Reference Include="System.Xml" />
     <Reference Include="System.Xml.Linq" />

IDisposableAnalyzers.Test/Properties/AssemblyInfo.cs

@@ -20,4 +20,5 @@
     typeof(Ninject.StandardKernel),
     typeof(Gu.Roslyn.AnalyzerExtensions.SyntaxTokenExt),
     typeof(Gu.Roslyn.CodeFixExtensions.Parse),
+    typeof(Stubs.Extensions),
     typeof(NUnit.Framework.Assert))]

IDisposableAnalyzers.sln

@@ -58,6 +58,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "IDisposableAnalyzers", "IDi
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "IDisposableAnalyzers.NetCoreTests", "IDisposableAnalyzers.NetCoreTests\IDisposableAnalyzers.NetCoreTests.csproj", "{8534B151-C831-4A31-B05B-7B2DAEA94033}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Stubs", "Stubs\Stubs.csproj", "{8CC16990-2621-4854-8556-ED648A2D360F}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU

IDisposableAnalyzers/Helpers/Disposable.IsCreation.cs

@@ -373,7 +373,7 @@ private static Result IsCreationCore(ISymbol candidate, Compilation compilation)
                         return Result.AssumeNo;
                     }
 
-                    if (method.TryGetThisParameter(out var thisParameter)&&
+                    if (method.TryGetThisParameter(out var thisParameter) &&
                         thisParameter.Type.Equals(method.ReturnType))
                     {
                         if (method.ReturnType == KnownSymbol.ILoggerFactory)

Stubs/Extensions.cs

@@ -0,0 +1,17 @@
+namespace Stubs
+{
+    using System;
+
+    public static class Extensions
+    {
+        /// <summary>
+        /// Test method that returns a different type than the in parameter. We assume that the method creates a new disposable then.
+        /// </summary>
+        public static ICustomDisposable AsCustom(this IDisposable disposable) => default(ICustomDisposable);
+
+        /// <summary>
+        /// Test method that returns the same type as the in parameter. We assume that the method does not create a new disposable then.
+        /// </summary>
+        public static IDisposable Fluent(this IDisposable disposable) => disposable;
+    }
+}

Stubs/ICustomDisposable.cs

@@ -0,0 +1,8 @@
+namespace Stubs
+{
+    using System;
+
+    public interface ICustomDisposable : IDisposable
+    {
+    }
+}

Stubs/Stubs.csproj

@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net461</TargetFramework>
+  </PropertyGroup>
+
+  <PropertyGroup>
+    <SignAssembly>true</SignAssembly>
+    <AssemblyOriginatorKeyFile>..\IDisposableAnalyzers.snk</AssemblyOriginatorKeyFile>
+  </PropertyGroup>
+</Project>