public interface IObjectService
{   
    object ReadWriteProperty { get; set; }

    object SingleArgReturnMethod(object arg);
}

using Mockup;
using Xunit;

// This will generate mock; use Mock(typeof(IObjectService)) for C# < 11.0
[assembly: Mock<IObjectService>]

namespace Mockup.Tests;

public class ObjectServiceMockTests
{
    [Fact]
    public void TestReadWriteProperty()
    {
        object value = "Value";
        
        var objectService = new ObjectServiceMock()
            .ReadWriteProperty(() => value, v => value = v)
            .Build(); // This will produce IObjectService
        
        // Your test code...
        var result = objectService.ReadWriteProperty; // returns "Value"
    }

    [Fact]
    public void TestSingleArgReturnMethod()
    {
        var objectService = new ObjectServiceMock()
            .SingleArgReturnMethod(v => "Changed" + v)
            .Build(); // This will produce IObjectService

        // Your test code...
        var result = objectService.SingleArgReturnMethod("Value"); // returns "ChangedValue"
    }
}