Don't use variable name from .pdb file if it isn't a valid C# identifier.

ICSharpCode.Decompiler/Ast/NameVariables.cs

@@ -62,7 +62,7 @@ namespace ICSharpCode.Decompiler.Ast
 					nv.AddExistingName(v.Name);
 				} else if (v.OriginalVariable != null && context.Settings.UseDebugSymbols) {
 					string varName = v.OriginalVariable.Name;
-					if (string.IsNullOrEmpty(varName) || varName.StartsWith("V_", StringComparison.Ordinal) || varName.StartsWith("CS$", StringComparison.Ordinal)) 
+					if (string.IsNullOrEmpty(varName) || varName.StartsWith("V_", StringComparison.Ordinal) || !IsValidName(varName))
 					{
 						// don't use the name from the debug symbols if it looks like a generated name
 						v.Name = null;
@@ -86,6 +86,19 @@ namespace ICSharpCode.Decompiler.Ast
 			}
 		}
 		
+		static bool IsValidName(string varName)
+		{
+			if (string.IsNullOrEmpty(varName))
+				return false;
+			if (!(char.IsLetter(varName[0]) || varName[0] == '_'))
+				return false;
+			for (int i = 1; i < varName.Length; i++) {
+				if (!(char.IsLetterOrDigit(varName[i]) || varName[i] == '_'))
+					return false;
+			}
+			return true;
+		}
+		
 		DecompilerContext context;
 		List<string> fieldNamesInCurrentType;
 		Dictionary<string, int> typeNames = new Dictionary<string, int>();