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Compiling a contract sample

So far, we have learned how to build a private chain and connect nodes to the chain. The following part will proceed to environment configuration, Neo smart contract coding and compilation and Neo smart contract deployment and invocation on private chain using C# and Windows 10.

We will complete the following tasks in this section:

  1. Install contract development environment

  2. Create a NEP-5 contract project

  3. Compile a contract

Installing development environment

Install Visual Studio 2019

Download and install Visual Studio 2019 . Select .NET Core cross-platform development option during installation.

Install NeoContractPlugin

Open Visual Studio 2019 and click EXTENSIONS -> Manage Extensions ,click Online on the left column, search "Neo" and install NeoContractPlugin (the process must be completed online).

3_download_and_install_smart_contract_plugin

Configure compiler

  1. Pull neo-devpack-dotnet project from GitHub to your localhost and open it.

  2. Switch the branch to master-2.x (Currently the master branch is the Neo3 compiler)

  3. Publish the project Neo.Compiler.MSIL

  4. After successful publish, you can find neon.exe under the path ...\Neo.Compiler.MSIL\bin\Release\netcoreapp3.1\publish\

Change environment parameter settings

Next we need to add path using the following method to allow neon.exe to be accessible from any point:

  1. For Windows 10, press Windows+S, input environment parameter and select Edit the system enviroment variables .

  2. Select Path and click edit :

  3. Click create in the popped up window and input your file filer directory that contains neon.exe, then press confirm .

Do not add a path that contains“…… neon.exe” in the environment parameter field. Remember to input the path of the file folder directory that contains neon.exe instead of the path of neon.exe file.

After the path is added, run CMD or PowerShell for testing purpose (if CMD starts working before the path is added, remember to restart it after adding the path). If no error is reported after inputting neon and the system sends the prompt message containing version number as follows, it means that the environment parameter configuration is successful.

3_1545037391347

Creating a Neo contract project

Upon completion of the previous steps, you may start to create Neo smart contract project in Visual Studio (no specific requirement for .NET Framework version):

  1. Click file -> create -> project .

  2. Select NeoContract in the list and change settings where necessary, then click confirm .

3_new_smart_contract_project

A C# file will be auto-generated after the project is created with a default class inherited from the SmartContract. As indicated in the screenshot below, now you have a Hello World contract.

3_smart_contract_function_code

Nevertheless, the above only demonstrates a simple data storage method - to store data in private storage area using key-value method.

Editing NEP-5 code

Many developers are curious about how to release their own contract assets on Neo public chain. Now let's walk through the process on private chain.

  1. Download the NEP-5 template from Github .

  2. Create a Neo smart contract project in Visual Studio and name it NEP5.

  3. Open NEP5.cs

    The code contains basic information of the assets and the methods available to be invoked. You can make changes when needed.

    If there are red underlines under the code warning that the system is unable to find Neo name space and there is "!" in project references, you may take the following steps:

    Right click the solution file in VS, click Manage NuGet Packages... and update the Neo.SmartContract.Framework to the latest official version in a new page. If the red underlines still exist when program update is completed, you may try double clicking the "!". If the problem remains unsolved, you may resort to the solutions below:

    1. Download nuget.exe here and copy it to the root directory of NeoContract project.

    2. Open Power Shell or command prompt (CMD).

    3. Redirect to the root directory of NeoContract project and run nuget restore .

  4. Here we make certain modifications to the example files as follows:

    • Define total assets and deploy method.

    • Replace "Owner" with the address in 0.json (otherwise the wallet assets won't be accessible).

    The code is as follows:

using Neo.SmartContract.Framework;
using Neo.SmartContract.Framework.Services.Neo;
using Neo.SmartContract.Framework.Services.System;
using System;
using System.ComponentModel;
using System.Numerics;

namespace NEP5
{
    public class NEP5 : SmartContract
    {
        [DisplayName("transfer")]
        public static event Action<byte[], byte[], BigInteger> Transferred;

        private static readonly byte[] Owner = "Ad1HKAATNmFT5buNgSxspbW68f4XVSssSw".ToScriptHash(); //Owner Address
        private static readonly BigInteger TotalSupplyValue = 10000000000000000;

        public static object Main(string method, object[] args)
        {
            if (Runtime.Trigger == TriggerType.Verification)
            {
                return Runtime.CheckWitness(Owner);
            }
            else if (Runtime.Trigger == TriggerType.Application)
            {
                var callscript = ExecutionEngine.CallingScriptHash;

                if (method == "balanceOf") return BalanceOf((byte[])args[0]);

                if (method == "decimals") return Decimals();

                if (method == "deploy") return Deploy();

                if (method == "name") return Name();

                if (method == "symbol") return Symbol();

                if (method == "supportedStandards") return SupportedStandards();

                if (method == "totalSupply") return TotalSupply();

                if (method == "transfer") return Transfer((byte[])args[0], (byte[])args[1], (BigInteger)args[2], callscript);
            }
            return false;
        }

        [DisplayName("balanceOf")]
        public static BigInteger BalanceOf(byte[] account)
        {
            if (account.Length != 20)
                throw new InvalidOperationException("The parameter account SHOULD be 20-byte addresses.");
            StorageMap asset = Storage.CurrentContext.CreateMap(nameof(asset));
            return asset.Get(account).AsBigInteger();
        }
        [DisplayName("decimals")]
        public static byte Decimals() => 8;

        private static bool IsPayable(byte[] to)
        {
            var c = Blockchain.GetContract(to);
            return c == null || c.IsPayable;
        }

        [DisplayName("deploy")]
        public static bool Deploy()
        {
            if (TotalSupply() != 0) return false;
            StorageMap contract = Storage.CurrentContext.CreateMap(nameof(contract));
            contract.Put("totalSupply", TotalSupplyValue);
            StorageMap asset = Storage.CurrentContext.CreateMap(nameof(asset));
            asset.Put(Owner, TotalSupplyValue);
            Transferred(null, Owner, TotalSupplyValue);
            return true;
        }

        [DisplayName("name")]
        public static string Name() => "GinoMo"; //name of the token

        [DisplayName("symbol")]
        public static string Symbol() => "GM"; //symbol of the token

        [DisplayName("supportedStandards")]
        public static string[] SupportedStandards() => new string[] { "NEP-5", "NEP-7", "NEP-10" };

        [DisplayName("totalSupply")]
        public static BigInteger TotalSupply()
        {
            StorageMap contract = Storage.CurrentContext.CreateMap(nameof(contract));
            return contract.Get("totalSupply").AsBigInteger();
        }
#if DEBUG
        [DisplayName("transfer")] //Only for ABI file
        public static bool Transfer(byte[] from, byte[] to, BigInteger amount) => true;
#endif
        //Methods of actual execution
        private static bool Transfer(byte[] from, byte[] to, BigInteger amount, byte[] callscript)
        {
            //Check parameters
            if (from.Length != 20 || to.Length != 20)
                throw new InvalidOperationException("The parameters from and to SHOULD be 20-byte addresses.");
            if (amount <= 0)
                throw new InvalidOperationException("The parameter amount MUST be greater than 0.");
            if (!IsPayable(to))
                return false;
            if (!Runtime.CheckWitness(from) && from.AsBigInteger() != callscript.AsBigInteger())
                return false;
            StorageMap asset = Storage.CurrentContext.CreateMap(nameof(asset));
            var fromAmount = asset.Get(from).AsBigInteger();
            if (fromAmount < amount)
                return false;
            if (from == to)
                return true;

            //Reduce payer balances
            if (fromAmount == amount)
                asset.Delete(from);
            else
                asset.Put(from, fromAmount - amount);

            //Increase the payee balance
            var toAmount = asset.Get(to).AsBigInteger();
            asset.Put(to, toAmount + amount);

            Transferred(from, to, amount);
            return true;
        }
    }
}

When the editing is done, the coding part of the smart contract is done.

Compiling contract file

Click BUILD -> Build Solutions (hotkeys: Ctrl + Shift + B) in the menu to start compilation.

When the compilation is done, Neo smart contract file named NeoContract1.avm is generated in the bin/Debug directory of the project.

NeoContract1.abi.json is a descriptive file of the smart contract, which contains desciptions of the ScriptHash, entry, parameters and return values of the contract. More information about the smart contract ABI can be found in NeoContract ABI .