Addition operators - + and +=
The + and += operators are supported by the built-in integral and floating-point numeric types, the string type, and delegate types.
For information about the arithmetic + operator, see the Unary plus and minus operators and Addition operator + sections of the Arithmetic operators article.
String concatenation
When one or both operands are of type string, the + operator concatenates the string representations of its operands (the string representation of null is an empty string):
Console.WriteLine("Forgot" + "white space");
Console.WriteLine("Probably the oldest constant: " + Math.PI);
Console.WriteLine(null + "Nothing to add.");
// Output:
// Forgotwhite space
// Probably the oldest constant: 3.14159265358979
// Nothing to add.
String interpolation provides a more convenient way to format strings:
Console.WriteLine($"Probably the oldest constant: {Math.PI:F2}");
// Output:
// Probably the oldest constant: 3.14
You can use string interpolation to initialize a constant string when all the expressions used for placeholders are also constant strings.
Beginning with C# 11, the + operator performs string concatenation for UTF-8 literal strings. This operator concatenates two ReadOnlySpan<byte> objects.
Delegate combination
For operands of the same delegate type, the + operator returns a new delegate instance that, when invoked, invokes the left-hand operand and then invokes the right-hand operand. If any of the operands is null, the + operator returns the value of another operand (which also might be null). The following example shows how delegates can be combined with the + operator:
Action a = () => Console.Write("a");
Action b = () => Console.Write("b");
Action ab = a + b;
ab(); // output: ab
To perform delegate removal, use the - operator.
For more information about delegate types, see Delegates.
Addition assignment operator +=
An expression using the += operator, such as
x += y
Is equivalent to:
x = x + y
Except that x is only evaluated once.
The following example demonstrates the usage of the += operator:
int i = 5;
i += 9;
Console.WriteLine(i);
// Output: 14
string story = "Start. ";
story += "End.";
Console.WriteLine(story);
// Output: Start. End.
Action printer = () => Console.Write("a");
printer(); // output: a
Console.WriteLine();
printer += () => Console.Write("b");
printer(); // output: ab
You also use the += operator to specify an event handler method when you subscribe to an event. For more information, see How to: subscribe to and unsubscribe from events.
Operator overloadability
A user-defined type can overload the + operator. When a binary + operator is overloaded, the += operator is also implicitly overloaded. A user-defined type can't explicitly overload the += operator.
C# language specification
For more information, see the Unary plus operator and Addition operator sections of the C# language specification.
