C# how do I use if in foreach

Quote:

when button6 clicked, there only a.exe added to listbox

The behavior is perfectly expected if second md5 is not what you expect.
We can't do anything for you, only the debugger can help you.

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

Debugging C# Code in Visual Studio - YouTube[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

It's definitely NOT because you're using an IF in a FOREACH. There is nothing wrong, nor special about an IF being used in a FOREACH block.

Use the debugger and step through the code line by line and examine the content of the variables. I'm sure you'll find the values in the variables are not exactly as you expect and that's why you're getting only one value added to your listbox.

Try moving the Try-Catch inside the ;foreach loop:

private void Button6_Click(object sender, EventArgs e)
{
    string[] source = new string[] { "8A1C8273F25E920CA8809107B069AC8D", "A8EF444D951FA378479A7C9D9611B6CC"};

    //string vr = 
    var files = System.IO.Directory.GetFiles(Environment.GetFolderPath(Environment.SpecialFolder.Desktop));

    foreach(var vr in files)
    {
        try
       { 
            var md5String = CreateMD5StringFromFile(vr); textBox1.Text = md5String.ToString();

            if (source.Contains(md5String))
            {
                listBox1.Items.Add(vr);
            }
        }                 
        catch (Exception ex)
        {
            Console.WriteLine($"error in file: {vr.FullName} | {ex.Message}");
        };
    }
}

You can run this as a "diagnostic" to see the errors that may be happening. After that, you should write specific handlers for the Types of errors thrown. I really agree with "never swallow an error you have not written an explicit catch for" ... except in debugging.

This thread on StackOverFlow has some interesting examples of newer ways in C#6 and #7 for handling catching multiple error types: [^]