VB.NET conversion to C #

That code is very bad, I think the best way to do this would be with if :

var WebReq = HttpWebRequest.Create(GET_Data);
using (var WebRes = WebReq.GetResponse)
using (var Reader = new StreamReader(WebRes.GetResponseStream)) {
    var Str = Reader.ReadLine;
    if (Str.Contains(Answer1)) return "True";
    if (Str.Contains(Answer2)) return "Banned";
    if (Str.Contains(Answer3)) return "Invalid";
    return "Invalid";
}

I placed it on GitHub for future reference.

Switch should only be used when you have a variable with multiple values that need to be tested. This is a case where you have multiple variables to test a single value, which is the case for if .

In your attempt while does not make sense, even less .ToString()!= null , this was not in the original. If you want to do something different you would have to say what. In fact if the variable Answer was an array another enumerable type, it would be possible to use while to make the 3 comparisons in a single if . Something tells me that this code should look very different from what was put.

I took advantage of and improved some more secondary things.

See this also: Naming Pattern in Code for C #

This construction of VB will not really work with C# , you will even need to use if if you want to test using Contains in this way. You can adapt the code using an expression to evaluate Contains before switch , something like this:

// Precisa adicionar esses using no começo do método
using System.Linq;
using System.Collections.Generic;

// Aqui vai o código:
string Str = Reader.ReadLine();
// Adicionar as strings que quer comparar em subStrings
List<string> subStrings = new List<string> { Answer1, Answer2, Answer3 };
switch (subStrings.FirstOrDefault(Str.Contains))
{
    case Answer1:
        Console.WriteLine("True");
        break;
    case Answer2:
        Console.WriteLine("Banned");
        break;
    case Answer3:
        Console.WriteLine("Invalid");
        break;
    default:
        Console.WriteLine("Invalid");
        break;
} 

You can use the switch() operator:

{
    HttpWebRequest WebReq = HttpWebRequest.Create(GET_Data);
    using (HttpWebResponse WebRes = WebReq.GetResponse)
    {
        using (StreamReader Reader = new StreamReader(WebRes.GetResponseStream))
        {
            string Str = Reader.ReadLine;
            switch (true)
            {
                case Str.Contains(Answer1):
                    return "True";
                case Str.Contains(Answer2):
                    return "Banned";
                case Str.Contains(Answer3):
                    return "Invalid";
                default:
                    return "Invalid";
            }
        }
    }
}