c#.NET and sprintf syntax

Check out string.Format and here is a version of your code using it:

string output = "The user {0} logged in";
string loggedIn = "is";
string loggedOut = "isn't";

if (TheUser.CheckStatus())
{
    output = string.Format(output, loggedIn);
}
else
{
    output = string.Format(output, loggedOut);
}

return output;

Or more simply: (using a ternary expression)

string output = "The user {0} logged in";

return TheUser.CheckStatus() 
    ? string.Format(output, "is")
    : string.Format(output, "isn't");

If you want to stick with %s,%d....

string sprintf(string input,params object[] inpVars)
{
    int i=0;
    input=Regex.Replace(input,"%.",m=>("{"+ i++/*increase have to be on right side*/ +"}"));
    return string.Format(input,inpVars);
}

You can now do

sprintf("hello %s..Hi %d","foofoo",455);

The whole printf family of functions in C is replaced by String.Format. The same interface is also exposed in for example Console.WriteLine().

 string output = "The user {0} logged in";
 string loggedIn = "is";
 string loggedOut = "isn't";


 output = string.Format(output, loggedIn);

With C# 6 you can use the formattable string:

if (TheUser.CheckStatus())
{
    output = $"The user {loggedIn} logged in"
}

The {loggedIn} inside the string is the variable name that you have defined.

Also, you have intellisense inside the curly braces to pick the variable name.