How do I write to the Visual Studio Output Window in My Custom Tool?

If you want anything to appear in the Output window, it has to come from stdout. To do this, your app needs to be linked as a "console" app. Set the /SUBSYSTEM:CONSOLE flag in the project's property page, under Linker/System set the SubSystem property to CONSOLE.

Once you have your output in the window, if you include the text "Error:" it will appear as an error, or if you set "Warning:" it will appear as a warning. If your error text begins with a path/filename, followed by a line number in parenthesis, the IDE will recognize it as a "clickable" error, and navigate you automatically to the faulting line.

This is demonstrated in the following helper class from a Microsoft sample project:

https://github.com/microsoft/VSSDK-Extensibility-Samples/blob/df10d37b863feeff6e8fcaa6f4d172f602a882c5/Reference_Services/C%23/Reference.Services/HelperFunctions.cs#L28

The code is as follows:

using System;
using System.Diagnostics;
using Microsoft.VisualStudio.Shell.Interop;

namespace Microsoft.Samples.VisualStudio.Services
{
    /// <summary>
    /// This class is used to expose some utility functions used in this project.
    /// </summary>
    internal static class HelperFunctions
    {
        /// <summary>
        /// This function is used to write a string on the Output window of Visual Studio.
        /// </summary>
        /// <param name="provider">The service provider to query for SVsOutputWindow</param>
        /// <param name="text">The text to write</param>
        internal static void WriteOnOutputWindow(IServiceProvider provider, string text)
        {
            // At first write the text on the debug output.
            Debug.WriteLine(text);

            // Check if we have a provider
            if (null == provider)
            {
                // If there is no provider we can not do anything; exit now.
                Debug.WriteLine("No service provider passed to WriteOnOutputWindow.");
                return;
            }

            // Now get the SVsOutputWindow service from the service provider.
            IVsOutputWindow outputWindow = provider.GetService(typeof(SVsOutputWindow)) as IVsOutputWindow;
            if (null == outputWindow)
            {
                // If the provider doesn't expose the service there is nothing we can do.
                // Write a message on the debug output and exit.
                Debug.WriteLine("Can not get the SVsOutputWindow service.");
                return;
            }

            // We can not write on the Output window itself, but only on one of its panes.
            // Here we try to use the "General" pane.
            Guid guidGeneral = Microsoft.VisualStudio.VSConstants.GUID_OutWindowGeneralPane;
            IVsOutputWindowPane windowPane;
            if (Microsoft.VisualStudio.ErrorHandler.Failed(outputWindow.GetPane(ref guidGeneral, out windowPane)) ||
                (null == windowPane))
            {
                if (Microsoft.VisualStudio.ErrorHandler.Failed(outputWindow.CreatePane(ref guidGeneral, "General", 1, 0)))
                {
                    // Nothing to do here, just debug output and exit
                    Debug.WriteLine("Failed to create the Output window pane.");
                    return;
                }
                if (Microsoft.VisualStudio.ErrorHandler.Failed(outputWindow.GetPane(ref guidGeneral, out windowPane)) ||
                (null == windowPane))
                {
                    // Again, there is nothing we can do to recover from this error, so write on the
                    // debug output and exit.
                    Debug.WriteLine("Failed to get the Output window pane.");
                    return;
                }
                if (Microsoft.VisualStudio.ErrorHandler.Failed(windowPane.Activate()))
                {
                    Debug.WriteLine("Failed to activate the Output window pane.");
                    return;
                }
            }

            // Finally we can write on the window pane.
            if (Microsoft.VisualStudio.ErrorHandler.Failed(windowPane.OutputString(text)))
            {
                Debug.WriteLine("Failed to write on the Output window pane.");
            }
        }
    }
}

You can use the Debug and/or Trace classes. There is some information here: http://msdn.microsoft.com/en-us/library/bs4c1wda(VS.71).aspx

Best of luck.

use System.Diagnostics.Debugger.Message