Added tests + bug fix

Author: vweijsters

StyleCop.Analyzers/StyleCop.Analyzers.Test/NamingRules/SA1312UnitTests.cs

@@ -274,6 +274,28 @@ public void MethodName(int parameter)
             await this.VerifyCSharpFixAsync(testCode, fixedCode).ConfigureAwait(false);
         }
 
+        [Fact]
+        public async Task TestUnderscoreOnlyNamesDoNotTriggerCodeFixAsync()
+        {
+            var testCode = @"public class TypeName
+{
+    public void MethodName(int parameter)
+    {
+        string _ = parameter.ToString();
+        string __ = parameter.ToString();
+    }
+}";
+
+            DiagnosticResult[] expected =
+            {
+                this.CSharpDiagnostic().WithArguments("_").WithLocation(5, 16),
+                this.CSharpDiagnostic().WithArguments("__").WithLocation(6, 16)
+            };
+
+            await this.VerifyCSharpDiagnosticAsync(testCode, expected, CancellationToken.None).ConfigureAwait(false);
+            await this.VerifyCSharpFixAsync(testCode, testCode).ConfigureAwait(false);
+        }
+
         protected override IEnumerable<DiagnosticAnalyzer> GetCSharpDiagnosticAnalyzers()
         {
             yield return new SA1312VariableNamesMustBeginWithLowerCaseLetter();

StyleCop.Analyzers/StyleCop.Analyzers.Test/NamingRules/SA1313UnitTests.cs

@@ -434,6 +434,7 @@ public void MethodName()
                 this.CSharpDiagnostic().WithArguments("__").WithLocation(7, 51)
             };
             await this.VerifyCSharpDiagnosticAsync(testCode, expected, CancellationToken.None).ConfigureAwait(false);
+            await this.VerifyCSharpFixAsync(testCode, testCode).ConfigureAwait(false);
         }
 
         /// <summary>

StyleCop.Analyzers/StyleCop.Analyzers/NamingRules/RenameToLowerCaseCodeFixProvider.cs

@@ -49,8 +49,13 @@ public override async Task RegisterCodeFixesAsync(CodeFixContext context)
                 if (!string.IsNullOrEmpty(token.ValueText))
                 {
                     var newName = token.ValueText.TrimStart('_');
-                    newName = char.ToLower(newName[0]) + newName.Substring(1);
-                    context.RegisterCodeFix(CodeAction.Create(string.Format(NamingResources.RenameToCodeFix, newName), cancellationToken => RenameHelper.RenameSymbolAsync(document, root, token, newName, cancellationToken), equivalenceKey: nameof(RenameToLowerCaseCodeFixProvider)), diagnostic);
+
+                    // only offer a codefix if the name does not consist of only underscores.
+                    if (!string.IsNullOrEmpty(newName))
+                    {
+                        newName = char.ToLower(newName[0]) + newName.Substring(1);
+                        context.RegisterCodeFix(CodeAction.Create(string.Format(NamingResources.RenameToCodeFix, newName), cancellationToken => RenameHelper.RenameSymbolAsync(document, root, token, newName, cancellationToken), equivalenceKey: nameof(RenameToLowerCaseCodeFixProvider)), diagnostic);
+                    }
                 }
             }
         }