Added some conversions

src/Neo.Compiler.MSIL/MSIL/Conv_Multi.cs

@@ -670,8 +670,7 @@ private int ConvertCall(OpCode src, NeoMethod to, List<MethodToken> methodTokens
                 }
                 else if (src.tokenMethod == "System.Char System.String::get_Chars(System.Int32)")
                 {
-                    ConvertPushNumber(1, src, to);
-                    Convert1by1(VM.OpCode.SUBSTR, null, to);
+                    Convert1by1(VM.OpCode.PICKITEM, src, to);
                     return 0;
                 }
                 else if (src.tokenMethod == "System.String System.String::Substring(System.Int32)")

src/Neo.SmartContract.Framework/Helper.cs

@@ -174,12 +174,22 @@ public static byte ToByte(this int source)
         [OpCode(OpCode.LEFT)]
         public extern static byte[] Take(this byte[] source, int count);
 
+        [OpCode(OpCode.SWAP)]
+        [OpCode(OpCode.LEFT)]
+        [OpCode(OpCode.CONVERT, StackItemType.ByteString)]
+        public extern static string Take(this string source, int count);
+
         /// <summary>
         /// Returns byte[] with last 'count' elements from 'source'. Faults if count < 0
         /// </summary>
         [OpCode(OpCode.RIGHT)]
         public extern static byte[] Last(this byte[] source, int count);
 
+        [OpCode(OpCode.SWAP)]
+        [OpCode(OpCode.RIGHT)]
+        [OpCode(OpCode.CONVERT, StackItemType.ByteString)]
+        public extern static string Last(this string source, int count);
+
         /// <summary>
         /// Returns a reversed copy of parameter 'source'.
         /// Example: [0a,0b,0c,0d,0e] -> [0e,0d,0c,0b,0a]

tests/Neo.Compiler.MSIL.UnitTests/TestClasses/Contract_Types_String.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Text;
+using System.Numerics;
+using System.Runtime.CompilerServices;
+using Neo.SmartContract.Framework;
+
+namespace Neo.Compiler.MSIL.UnitTests.TestClasses
+{
+    class Contract_Types_String : SmartContract.Framework.SmartContract
+    {
+        public static char checkIndex(string cad, int pos) { return cad[pos]; }
+        public static string checkRange(string cad, int pos, int count) { return cad.Substring(pos, count); }
+        public static string checkTake(string cad, int count) { return cad.Take(count); }
+        public static string checkLast(string cad, int count) { return cad.Last(count); }
+    }
+}

tests/Neo.Compiler.MSIL.UnitTests/UnitTest_Types.cs

@@ -521,5 +521,39 @@ public void UInt160_byte_array_construct()
             var received = new UInt160(((ByteString)item).GetSpan());
             Assert.AreEqual(received, notZero);
         }
+
+        [TestMethod]
+        public void String_Methods()
+        {
+            using var testengine = new TestEngine();
+            testengine.AddEntryScript("./TestClasses/Contract_Types_String.cs");
+
+            var result = testengine.ExecuteTestCaseStandard("checkIndex", "hello", 4);
+            Assert.AreEqual(1, result.Count);
+            var item = result.Pop();
+            Assert.IsTrue(item is Integer);
+            Assert.AreEqual((byte)'o', item.GetInteger());
+
+            testengine.Reset();
+            result = testengine.ExecuteTestCaseStandard("checkTake", "hello", 4);
+            Assert.AreEqual(1, result.Count);
+            item = result.Pop();
+            Assert.IsTrue(item is ByteString);
+            Assert.AreEqual("hell", item.GetString());
+
+            testengine.Reset();
+            result = testengine.ExecuteTestCaseStandard("checkLast", "hello", 2);
+            Assert.AreEqual(1, result.Count);
+            item = result.Pop();
+            Assert.IsTrue(item is ByteString);
+            Assert.AreEqual("lo", item.GetString());
+
+            testengine.Reset();
+            result = testengine.ExecuteTestCaseStandard("checkRange", "hello", 1, 2);
+            Assert.AreEqual(1, result.Count);
+            item = result.Pop();
+            Assert.IsTrue(item is ByteString);
+            Assert.AreEqual("el", item.GetString());
+        }
     }
 }