XUnit BeforeAfterTestAttribute how do I get runtime values of nameof parameters

This known pattern for BeforeAfterTestAttribute works as intended but i'm trying to do a bit extra on the after method. I'm trying to pass the value of a member which has just been "acted" in ProfileTests class, named json

//[WriteFileAfter("Company.json", nameof(json))]  //Not working
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
public class WriteFileAfter : BeforeAfterTestAttribute {
    public string FileName { get; }
    public string Json { get; }

    public WriteFileAfter(string fileName, string json) {
        FileName = fileName;
        Json = json;
    }

    //public override void Before(MethodInfo methodUnderTest) { //ignore
    //    Debug.WriteLine(methodUnderTest.Name); }

    public override void After(MethodInfo methodUnderTest) {
        Debug.WriteLine(methodUnderTest.Name);
        JsonUtils.WriteJsonToFile(**Json**, methodUnderTest.Name); 
                                   //^-- Json = 'json'
    }
}

The following code shows me using this attribute and passing "json" which is the nameof json, yes I understand it's just 'json'

public class ProfileTests
{ 
    public string json; 

    public ProfileTests()
    {
    }

    [Theory]
    [InlineData(@"test.txt")]
    [WriteFileAfter("Company.json", nameof(json))]
    public void MyTest(string fileName) {
        // Arrange
        // Act
        // Assert
 
        //Output?
        //json = JsonConvert.SerializeObject(package);
        json = "Hello World"
    }

}

How do I reflect back to the "value" of json so I can pass it to JsonUtils.WriteJsonToFile method because I know the name of the attribute I want the value. I've tried various reflection hacks (no expert) and can't seem to grasp how to do this. Thanks.

nameof(json) will return 'json' as expected but I wan the value I've google a bit but can't figure out the correct syntax.