Add RuleVersionAttribute

runtime/CSharp/Antlr4.Runtime/Antlr4.Runtime.csproj

@@ -145,6 +145,7 @@
     <Compile Include="RecognitionException.cs" />
     <Compile Include="Recognizer`2.cs" />
     <Compile Include="RuleContext.cs" />
+    <Compile Include="RuleVersionAttribute.cs" />
     <Compile Include="Sharpen\Arrays.cs" />
     <Compile Include="Sharpen\AtomicReference`1.cs" />
     <Compile Include="Sharpen\BitSet.cs" />

runtime/CSharp/Antlr4.Runtime/RuleVersionAttribute.cs

@@ -0,0 +1,23 @@
+namespace Antlr4.Runtime
+{
+    using System;
+
+    [AttributeUsage(AttributeTargets.Method, Inherited = false, AllowMultiple = false)]
+    public sealed class RuleVersionAttribute : Attribute
+    {
+        private readonly int _version;
+
+        public RuleVersionAttribute(int version)
+        {
+            _version = version;
+        }
+
+        public int Version
+        {
+            get
+            {
+                return _version;
+            }
+        }
+    }
+}