Updated SA1309CodeFixProvider to not provide a code fix if the resulting identifier would be illegal.

Author: bjornhellander

StyleCop.Analyzers/StyleCop.Analyzers.CodeFixes/NamingRules/SA1309CodeFixProvider.cs

@@ -10,6 +10,7 @@ namespace StyleCop.Analyzers.NamingRules
     using Microsoft.CodeAnalysis;
     using Microsoft.CodeAnalysis.CodeActions;
     using Microsoft.CodeAnalysis.CodeFixes;
+    using Microsoft.CodeAnalysis.CSharp;
 
     /// <summary>
     /// Implements a code fix for <see cref="SA1309FieldNamesMustNotBeginWithUnderscore"/>.
@@ -45,10 +46,9 @@ public override async Task RegisterCodeFixesAsync(CodeFixContext context)
                 {
                     var newName = token.ValueText.TrimStart(new[] { '_' });
 
-                    if (string.IsNullOrEmpty(newName))
+                    if (!SyntaxFacts.IsValidIdentifier(newName))
                     {
-                        // The variable consisted of only underscores. In this case we cannot
-                        // generate a valid variable name and thus will not offer a code fix.
+                        // The proposed name was not legal, so no code fix will be offered.
                         continue;
                     }
 

StyleCop.Analyzers/StyleCop.Analyzers.Test/NamingRules/SA1309UnitTests.cs

@@ -132,6 +132,23 @@ public async Task TestFieldStartingWithMultipleUnderscoresAsync()
             await this.VerifyCSharpFixAsync(testCode, fixedCode).ConfigureAwait(false);
         }
 
+        // This a regression test for issue #2360.
+        [Fact]
+        public async Task VerifyThatAFieldStartingWithAnUnderscoreAndADigitIsNotAffectedByCodeFixAsync()
+        {
+            var testCode = @"public class Foo
+{
+    private string _1bar = ""baz"";
+}";
+
+            DiagnosticResult expected = this.CSharpDiagnostic().WithArguments("_1bar").WithLocation(3, 20);
+            await this.VerifyCSharpDiagnosticAsync(testCode, expected, CancellationToken.None).ConfigureAwait(false);
+
+            // no changes will be made
+            var fixedCode = testCode;
+            await this.VerifyCSharpFixAsync(testCode, fixedCode).ConfigureAwait(false);
+        }
+
         protected override IEnumerable<DiagnosticAnalyzer> GetCSharpDiagnosticAnalyzers()
         {
             yield return new SA1309FieldNamesMustNotBeginWithUnderscore();