Implement NEP-17

src/neo/SmartContract/ApplicationEngine.Contract.cs

@@ -135,9 +135,17 @@ private void CallContractInternal(UInt160 contractHash, string method, Array arg
             if (method.StartsWith('_')) throw new ArgumentException($"Invalid Method Name: {method}");
 
             ContractState contract = Snapshot.Contracts.TryGet(contractHash);
-            if (contract is null) throw new InvalidOperationException($"Called Contract Does Not Exist: {contractHash}");
+            if (contract is null)
+            {
+                if (flags.HasFlag(CallFlags.IfExists)) return;
+                throw new InvalidOperationException($"Called Contract Does Not Exist: {contractHash}");
+            }
             ContractMethodDescriptor md = contract.Manifest.Abi.GetMethod(method);
-            if (md is null) throw new InvalidOperationException($"Method {method} Does Not Exist In Contract {contractHash}");
+            if (md is null)
+            {
+                if (flags.HasFlag(CallFlags.IfExists)) return;
+                throw new InvalidOperationException($"Method {method} Does Not Exist In Contract {contractHash}");
+            }
 
             ContractManifest currentManifest = Snapshot.Contracts.TryGet(CurrentScriptHash)?.Manifest;
             if (currentManifest != null && !currentManifest.CanCall(contract.Manifest, method))

src/neo/SmartContract/CallFlags.cs

@@ -12,6 +12,8 @@ public enum CallFlags : byte
         AllowCall = 0b00000100,
         AllowNotify = 0b00001000,
 
+        IfExists = 0b10000000,
+
         ReadOnly = AllowStates | AllowCall | AllowNotify,
         All = AllowStates | AllowModifyStates | AllowCall | AllowNotify
     }