Add Method (Int64, Int64)
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Interlocked.Add Method (Int64%, Int64)

Adds two 64-bit integers and replaces the first integer with the sum, as an atomic operation.

Namespace:  System.Threading
Assembly:  mscorlib (in mscorlib.dll)

public static long Add(
	ref long location1,
	long value


Type: System.Int64%

A variable containing the first value to be added. The sum of the two values is stored in location1.

Type: System.Int64

The value to be added to the integer at location1.

Return Value

Type: System.Int64
The new value stored at location1.


The address of location1 is a null pointer.

This method handles an overflow condition by wrapping: if the value at location1 is Int64.MaxValue and value is 1, the result is Int64.MinValue; if value is 2, the result is (Int64.MinValue + 1); and so on. No exception is thrown.

The Read method and the 64-bit overloads of the Increment, Decrement, and Add methods are truly atomic only on systems where a System.IntPtr is 64 bits long. On other systems, these methods are atomic with respect to each other, but not with respect to other means of accessing the data. Thus, to be thread safe on 32-bit systems, any access to a 64-bit value must be made through the members of the Interlocked class.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

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