How to convert a relative path to an absolute path in a Windows application?

Have you tried:

string absolute = Path.GetFullPath(relative);

? Note that that will use the current working directory of the process, not the directory containing the executable. If that doesn't help, please clarify your question.

If you want to get the path relative to your .exe then use

string absolute = Path.Combine(Application.ExecutablePath, relative);

This one works for paths on different drives, for drive-relative paths and for actual relative paths. Heck, it even works if the basePath isn't actually absolute; it always uses the current working directory as final fallback.

public static String GetAbsolutePath(String path)
{
    return GetAbsolutePath(null, path);
}

public static String GetAbsolutePath(String basePath, String path)
{
    if (path == null)
        return null;
    if (basePath == null)
        basePath = Path.GetFullPath("."); // quick way of getting current working directory
    else
        basePath = GetAbsolutePath(null, basePath); // to be REALLY sure ;)
    String finalPath;
    // specific for windows paths starting on \ - they need the drive added to them.
    // I constructed this piece like this for possible Mono support.
    if (!Path.IsPathRooted(path) || "\\".Equals(Path.GetPathRoot(path)))
    {
        if (path.StartsWith(Path.DirectorySeparatorChar.ToString()))
            finalPath = Path.Combine(Path.GetPathRoot(basePath), path.TrimStart(Path.DirectorySeparatorChar));
        else
            finalPath = Path.Combine(basePath, path);
    }
    else
        finalPath = path;
    // resolves any internal "..\" to get the true full path.
    return Path.GetFullPath(finalPath);
}

It's a bit older topic, but it might be useful for someone. I have solved a similar problem, but in my case, the path was not at the beginning of the text.

So here is my solution:

public static class StringExtension
{
    private const string parentSymbol = "..\\";
    private const string absoluteSymbol = ".\\";
    public static String AbsolutePath(this string relativePath)
    {
        string replacePath = AppDomain.CurrentDomain.BaseDirectory;
        int parentStart = relativePath.IndexOf(parentSymbol);
        int absoluteStart = relativePath.IndexOf(absoluteSymbol);
        if (parentStart >= 0)
        {
            int parentLength = 0;
            while (relativePath.Substring(parentStart + parentLength).Contains(parentSymbol))
            {
                replacePath = new DirectoryInfo(replacePath).Parent.FullName;
                parentLength = parentLength + parentSymbol.Length;
            };
            relativePath = relativePath.Replace(relativePath.Substring(parentStart, parentLength), string.Format("{0}\\", replacePath));
        }
        else if (absoluteStart >= 0)
        {
            relativePath = relativePath.Replace(".\\", replacePath);
        }
        return relativePath;
    }
}

Example:

Data Source=.\Data\Data.sdf;Persist Security Info=False;
Data Source=..\..\bin\Debug\Data\Data.sdf;Persist Security Info=False;