Using the PrivateObject class I can access private members of a class.
But I cannot access private members if that class is mocked using Moq.

Example Code

using System.Reflection;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Moq;
using Xunit;
using Assert = Xunit.Assert;

namespace MyNamespace;

public class Temp
{
    private int I { get; set; }

    public int PublicI
    {
        get => I;
        set => I = value;
    }

    public Temp(int i)
    {
        I = i;
    }
}

public class Testing
{
    [Fact]
    [Trait("Category", "Unit")]
    public void TestNormal()
    {
        var temp = new Temp(7);
        Assert.Equal(7, temp.PublicI);
        temp.PublicI = 8;
        Assert.Equal(8, temp.PublicI);

        var po = new PrivateObject(temp);
        var publicI = po.GetFieldOrProperty("PublicI");
        Assert.Equal(8, publicI);

        var privateI = po.GetFieldOrProperty("I", BindingFlags.NonPublic | BindingFlags.Instance);
        Assert.Equal(8, privateI);
    }

    [Fact]
    [Trait("Category", "Unit")]
    public void TestMocked()
    {
        var mock = new Mock<Temp>(7);
        Assert.Equal(7, mock.Object.PublicI);
        mock.Object.PublicI = 8;
        Assert.Equal(8, mock.Object.PublicI);

        var po = new PrivateObject(mock.Object);
        var publicI = po.GetFieldOrProperty("PublicI");
        Assert.Equal(8, publicI);

        // Following line throws System.MissingMethodException
        var privateI = po.GetFieldOrProperty("I", BindingFlags.NonPublic | BindingFlags.Instance);
        Assert.Equal(8, privateI);
    }
}

The line after the comment throws a MissingMethodException. Why doesn’t it find my member?