Showing how to mock a dependency

BigMethodClassTests.cs

using System;
using Moq;
using Xunit;

public class BigMethodClass
{

  private readonly IBitmapOperations _bitmapOperations;

  public BigMethodClass( IBitmapOperations bitmapOperations )
  {
    _bitmapOperations = bitmapOperations;
  }

  public int BigMethod()
  {
    _bitmapOperations.DoBitMapStuff();
    // other methods;
    return 1; //return whatever part your testing
  }
}

public interface IBitmapOperations
{
  void DoBitMapStuff();
}

public class BigMethodClassTests
{
  [Fact]
  public void TestBigMethodReturns1()
  {
    const int expected = 1;
    var bitmapOperationMoq = new Mock<IBitmapOperations>();
    var testing = new BigMethodClass( bitmapOperationMoq.Object );

    var actual = testing.BigMethod();
    Assert.Equal( expected: expected, actual: actual );
  }
}