How to redirect standard output for a .NET console application

Normally a .NET console application writes its messages directly to the console so that the user can view them. This is done with one of the “Write” methods like Console.WriteLine(some message) where you can prompt the user for some input.

However, this is not the only option you have. The standard output channel can be overridden.

All you need is an object which derives from the abstract TextWriter class. There are a couple of classes in .NET that derive from this base class, such as StringWriter which stores the text in a StringBuilder object and StreamWriter which saves the text to a file.

We’ll look at an example using StringWriter, i.e. we’ll tell the console to “write the lines” to a StringBuilder instead of the default console. The Console.SetOut method is the most important method to remember if you want to redirect the standard output.

The following example will generate 10 random strings with a small pause in between each random string. Standard output is redirected to a StringWriter hence Console.WriteLine won’t output the messages to the console but the StringBuilder that was added to the StringWriter constructor:

static void Main(string[] args)
{
	RunStandardOutRedirectExample();
}

private static void RunStandardOutRedirectExample()
{
	StringBuilder builder = new StringBuilder();
	TextWriter writer = new StringWriter(builder);
	Console.SetOut(writer);

	for (int i = 0; i < 10; i++)
	{
		Console.WriteLine(GetRandomString(5));
		Thread.Sleep(100);
	}

	string result = builder.ToString();
}

private static string GetRandomString(int size)
{
	Random random = new Random();
	string charSet = "abcdefghijklmnopqestuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
	char[] buffer = new char[size];
	for (int i = 0; i < size; i++)
	{
		buffer[i] = charSet[random.Next(charSet.Length)];
	}
	return new string(buffer);
}

Here’s an example of what “result” could look like:

FHIHZ
KlsKv
HkCYG
NOnbd
ednTd
QshsL
uHikL
eGeyW
xkcBs
CONFP

The StreamWriter class will help you save all that in a file instead.

If you’d like to reset Console.WriteLine to write the messages to the console then insert the following bit of code:

Console.SetIn(new StreamReader(Console.OpenStandardOutput()));

View all various C# language feature related posts here.