In a .cs file, you can call IsNullOrEmpty() directly on a string without any issue (StringVariable.IsNullOrEmpty()). However, in a .razor file, you have to write String.IsNullOrEmpty(StringVariable) in order to call the function. What is the cause of this difference? Why isn't it standardized, considering both are in c# and the babies of Microsoft?
Variable.IsNullOrEmpty() is not built into the language. Often many developers will implement it as an extension method (example) you may have tripped and fallen into one of situations where its included in a dependency and you have access to it.
public static class StringExtensions {
public bool IsNullOrEmpty(this string mystring) => String.IsNullOrEmpty(mystring);
}