StandardOutput.ReadToEnd() hangs [duplicate]

Question

I have a program that frequently uses an external program and reads its outputs. It works pretty well using your usual process redirect output, but one specific argument for some reason hangs when I try to read it, no error message - no exception, it just 'stops' when it reaches that line. I of course use a centralized function to call and read output from the program, which is this:

public string ADBShell(string adbInput)
{
    try
    {
        //Create Empty values
        string result = string.Empty;
        string error = string.Empty;
        string output = string.Empty;
        System.Diagnostics.ProcessStartInfo procStartInfo 
            = new System.Diagnostics.ProcessStartInfo(toolPath + "adb.exe");

        procStartInfo.Arguments = adbInput;
        procStartInfo.RedirectStandardOutput = true;
        procStartInfo.RedirectStandardError = true;
        procStartInfo.UseShellExecute = false;
        procStartInfo.CreateNoWindow = true;
        procStartInfo.WorkingDirectory = toolPath;
        System.Diagnostics.Process proc = new System.Diagnostics.Process();
        proc.StartInfo = procStartInfo;
        proc.Start();
        // Get the output into a string
        proc.WaitForExit();
        result = proc.StandardOutput.ReadToEnd();
        error = proc.StandardError.ReadToEnd();  //Some ADB outputs use this
        if (result.Length > 1)
        {
            output += result;
        }
        if (error.Length > 1)
        {
            output += error;
        }
        Return output;
    }
    catch (Exception objException)
    {
        throw objException;
    }
}

The line that hangs is result = proc.StandardOutput.ReadToEnd();, but again, not every time, only when sent a specific argument ("start-server"). All other arguments work just fine - it reads the value and returns it. It's also strange the way it hangs. It doesn't freeze or give an error or anything, it just stops processing. As if it was a 'return' command, except it doesn't even return to the calling function, it just stops everything with the interface still up and running. Anyone experienced this before? Anyone have any idea what I should try? I'm assuming it's something unexpected within the stream itself, but is there a way I can handle/ignore this so that it reads it anyway?

Answer 1

Proposed solutions with BeginOutputReadLine() are a good way but in situations such as that, it is not applicable, because process (certainly with using WaitForExit()) exits earlier than async output finished completely.

So, I tried to implement it synchronously and found that the solution is in using Peek() method from StreamReader class. I added check for Peek() > -1 to sure that it is not the end of the stream as in MSDN article described and finally it works and stop hanging!

Here is the code:

var process = new Process();
process.StartInfo.CreateNoWindow = true;
process.StartInfo.UseShellExecute = false;
process.StartInfo.RedirectStandardOutput = true;
process.StartInfo.RedirectStandardError = true;
process.StartInfo.WorkingDirectory = @"C:\test\";
process.StartInfo.FileName = "test.exe";
process.StartInfo.Arguments = "your arguments here";

process.Start();
var output = new List<string>();

while (process.StandardOutput.Peek() > -1)
{
    output.Add(process.StandardOutput.ReadLine());
}

while (process.StandardError.Peek() > -1)
{
    output.Add(process.StandardError.ReadLine());
}
process.WaitForExit();

Answer 2

The problem is that you are using the synchronous ReadToEnd methods on both the StandardOutput and the StandardError streams. This can lead to a potential deadlock you are experiencing. This is even described in the MSDN. The solution is described there. Basically, it is: Use the asynchronous version BeginOutputReadLine to read the data of the StandardOutput stream:

p.BeginOutputReadLine();
string error = p.StandardError.ReadToEnd();
p.WaitForExit();

Implementation of Async reading using BeginOutputReadLine see in ProcessStartInfo hanging on "WaitForExit"? Why?