EnsureEndsWith

/// <summary>
/// 	Ensures that a string ends with a given suffix.
/// </summary>
/// <param name = "value">The string value to check.</param>
/// <param name = "suffix">The suffix value to check for.</param>
/// <returns>The string value including the suffix</returns>
/// <example>
/// 	<code>
/// 		var url = "http://www.pgk.de";
/// 		url = url.EnsureEndsWith("/"));
/// 	</code>
/// </example>
public static string EnsureEndsWith(this string value, string suffix)
{
    return value.EndsWith(suffix) ? value : value.Insert(value.Length, suffix);
}

var sentence = "This is a sentence".EnsureEndsWith(".");