ECDSA with secp256k1 in C# - Generate Keys, Sign, Verify

ECDSA-secp256k1-example.cs

using System;
using System.Text;
using Nethereum.Hex.HexConvertors.Extensions;
using Nethereum.Signer;
using Nethereum.Util;
using Nethereum.Signer.Crypto;

class ECDSASecp256k1Example
{
    static void Main()
    {
        //var privKey = EthECKey.GenerateKey();
        var privKey = new EthECKey("97ddae0f3a25b92268175400149d65d6887b9cefaf28ea2c078e05cdc15a3c0a");
        byte[] pubKeyCompressed = new ECKey(privKey.GetPrivateKeyAsBytes(), true).GetPubKey(true);
        Console.WriteLine("Private key: {0}", privKey.GetPrivateKey().Substring(4));
        Console.WriteLine("Public key: {0}", privKey.GetPubKey().ToHex().Substring(2));
        Console.WriteLine("Public key (compressed): {0}", pubKeyCompressed.ToHex());

        Console.WriteLine();

        string msg = "Message for signing";
        byte[] msgBytes = Encoding.UTF8.GetBytes(msg);
        byte[] msgHash = new Sha3Keccack().CalculateHash(msgBytes);
        var signature = privKey.SignAndCalculateV(msgHash);
        Console.WriteLine("Msg: {0}", msg);
        Console.WriteLine("Msg hash: {0}", msgHash.ToHex());
        Console.WriteLine("Signature: [v = {0}, r = {1}, s = {2}]",
            signature.V[0] - 27, signature.R.ToHex(), signature.S.ToHex());

        Console.WriteLine();

        var pubKeyRecovered = EthECKey.RecoverFromSignature(signature, msgHash);
        Console.WriteLine("Recovered pubKey: {0}", pubKeyRecovered.GetPubKey().ToHex().Substring(2));

        bool validSig = pubKeyRecovered.Verify(msgHash, signature);
        Console.WriteLine("Signature valid? {0}", validSig);
    }
}

Secp256k1-ECDSA-CSharp.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Nethereum.Signer" Version="2.4.0" />
  </ItemGroup>

</Project>

where is EthECKey definition？

i see, Nethereum.Signer PackageReference