string.IsNullOrEmpty() / string.IsNullOrWhiteSpace()

// Returns true if the string is null or "".
string.IsNullOrEmpty(string? value)

// Returns true if the string is null, "", or contains only whitespace (spaces, tabs, etc.).
string.IsNullOrWhiteSpace(string? value)

// Returns the right-hand value if the left-hand side is null (null-coalescing operator).
left ?? right

// Assigns the right-hand value to the variable only if it is null (null-coalescing assignment operator).
variable ??= value

using System;

// How to use IsNullOrEmpty().
string? name1 = null;
string? name2 = "";
string? name3 = "Tanaka";

Console.WriteLine(string.IsNullOrEmpty(name1)); // True
Console.WriteLine(string.IsNullOrEmpty(name2)); // True
Console.WriteLine(string.IsNullOrEmpty(name3)); // False

// IsNullOrWhiteSpace() also detects strings that contain only spaces.
string? input = "   "; // Whitespace only.
Console.WriteLine(string.IsNullOrEmpty(input));      // False (IsNullOrEmpty does not catch this)
Console.WriteLine(string.IsNullOrWhiteSpace(input)); // True

// Use the ?? operator to provide a default value when the result is null.
string? username = null;
string displayName = username ?? "Guest";
Console.WriteLine(displayName); // Guest

string? setting = "Dark";
string theme = setting ?? "Light";
Console.WriteLine(theme); // Dark (not null, so the original value is used)

// Example of the ??= operator.
string? cache = null;
cache ??= "initial value";
Console.WriteLine(cache); // initial value

When validating form input, you need to account for users entering only spaces. In such cases, it is recommended to prefer IsNullOrWhiteSpace() over IsNullOrEmpty().

The ?? operator lets you simplify null-check if statements and is commonly used to set default values. For details on nullable types (string?), refer to the C# null-safety design specification.

For type conversion utilities, see 'Convert.ToString() / Convert.ToInt32()'.