CODEWITHSUNDEEP
c#process
gash25
gash25

Reputation: 307

Process.Start returns null

I'm writing a program that launches a random file in s directory. the file can be of any type, but mostly video or image files. Each time I launch a file I want to close the previous opened one.

Code :

string FolderSelected = "";
string FileName = "";
Process proc;
            
List<string> FilesDisplayed = new List<string>();

private void button2_Click(object sender, EventArgs e)
{
    if (FolderSelected == string.Empty)
        FolderSelected = Properties.Settings.Default.FilesDefaultFolder;

    if (proc != null)
    {
        proc.CloseMainWindow();
        proc.Close();
    }
    FileName = FetchRandomFile();
    proc = Process.Start(FileName);
}

Problem is, that I keep getting proc = null (the file is launched properly) and I cannot fetch the previously opened process in order to close it. I know that .NET reuses processes and that's why it returns Null but I need to override this behavior.

Upvotes: 21

Views: 12537

Answers (4)

Despertar
Despertar

Reputation: 22352

To fix this problem, you have to set UseShellExecute to false to bypass the shell.

Instead of

Process.Start("filename", "args")

use

Process.Start(new ProcessStartInfo() {
    FileName = "filename",
    Arguments = "args",
    UseShellExecute = false
});

Upvotes: 1

Jon Skeet
Jon Skeet

Reputation: 1499760

EDIT: Thanks to leppie's comment, I suspect I know the answer: my guess is that you're "starting" something like an image, and it's reusing an existing process to open the document instead of creating a new one.

I've reproduced this with this simple test app:

using System;
using System.Diagnostics;

public class Test
{
    static void Main()
    {
        Process proc = Process.Start("image.tif");
        Console.WriteLine(proc == null);
    }
}

This prints "true" because it's using dllhost.exe to host the Windows Image Viewer, rather than creating a new process.

Upvotes: 16

Marc Gravell
Marc Gravell

Reputation: 1062492

That won't even compile (definite assignment). As a method variable, proc is local only to the declaring method(/scope) - i.e. button2_Click, which explains why you can't retain values. If proc is meant to persist between calls, promote it to a field (per-instance variable):

Process proc;
private void button2_Click(object sender, EventArgs e)
{
    if (proc != null)
    ...

Upvotes: 0

Ed Swangren
Ed Swangren

Reputation: 124622

You are declaring 'proc' at method scope, so of course it will always be null when checked at the top of that method. If you want the reference to live beyond the function, declare it is a class level variable.

You are spawning a process each time (probably), Process.Start is not returning null, but you simply lose the reference to it when i goes out of scope.

Upvotes: 0

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