Volatile Class

Definition

Namespace:

System.Threading

Assembly:

System.Threading.dll

Source:

Volatile.cs

Contains methods for performing volatile memory operations.

public ref class Volatile abstract sealed

public static class Volatile

type Volatile = class

Public Class Volatile

Inheritance

Object

Volatile

Remarks

On a multiprocessor system, due to performance optimizations in the compiler or processor, regular memory operations may appear to be reordered when multiple processors are operating on the same memory. Volatile memory operations prevent certain types of reordering with respect to the operation. A volatile write operation prevents earlier memory operations on the thread from being reordered to occur after the volatile write. A volatile read operation prevents later memory operations on the thread from being reordered to occur before the volatile read. These operations might involve memory barriers on some processors, which can affect performance.

For example, consider the following scenario with two threads and two Int32 fields x and y that are initially zero:

Thread 1	Thread 2
x = 1;	int y2 = Volatile.Read(ref y);
Volatile.Write(ref y, 1);	int x2 = x;

The volatile read and write prevent reordering of the two operations within each thread, such as by the compiler or by the processor. Regardless of the order in which these operations actually occur on one thread relative to the other thread, even on a multiprocessor system where the threads may run on different processors, the volatile operations guarantee that thread 2 would not see y2 == 1 and x2 == 0. On thread 1 the write to x must appear to occur before the volatile write to y, and on thread 2 the read of x must appear to occur after the volatile read of y. So if thread 2 sees y2 == 1, it must also see x2 == 1.

However, consider the same scenario as above with a specific sequence in which the operations occur:

Sequence	Thread 1	Thread 2
1	x = 1;	...
2	Volatile.Write(ref y, 1);	...
3	...	int y2 = Volatile.Read(ref y);
4	...	int x2 = x;

Even though the volatile write to y on thread 1 occurred before the volatile read of y on thread 2, thread 2 may still see y2 == 0. The volatile write to y does not guarantee that a following volatile read of y on a different processor will see the updated value.

Note

• Volatile reads and writes ensure that a value is read or written to memory and not cached (for example, in a processor register). Thus, you can use these operations to synchronize access to a field that can be updated by another thread or by hardware.
• The Volatile class also provides read and write operations for some 64-bit types such as Int64 and Double. Volatile reads and writes on such 64-bit memory are atomic even on 32-bit processors, unlike regular reads and writes.

Volatile memory operations are for special cases of synchronization, where normal locking is not an acceptable alternative. Under normal circumstances, the C# lock statement, the Visual Basic SyncLock statement, and the Monitor class provide the easiest and least error-prone way of synchronizing access to data, and the Lazy<T> class provides a simple way to write lazy initialization code without directly using double-checked locking.

The Volatile.Read and Volatile.Write methods enable functionality that is not supported in languages. For example:

• Some languages, such as Visual Basic, do not recognize the concept of volatile memory operations. The Volatile class provides that functionality in such languages.

  Note

  Calling one of these methods affects only a single memory access. To provide effective synchronization for a field, all access to the field must use Volatile.Read and Volatile.Write.

• In C#, using the volatile modifier on a field guarantees that every access to that field is a volatile memory operation, but the volatile modifier cannot be applied to array elements. The Volatile.Read and Volatile.Write methods can be used on array elements.

Methods

Read(Boolean)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(Byte)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(Double)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(Int16)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(Int32)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(Int64)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(IntPtr)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(SByte)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(Single)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(UInt16)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(UInt32)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(UInt64)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read(UIntPtr)	Reads the value of the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Read<T>(T)	Reads the object reference from the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears after this method in the code, the processor cannot move it before this method.
Write(Boolean, Boolean)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(Byte, Byte)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(Double, Double)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(Int16, Int16)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(Int32, Int32)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(Int64, Int64)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(IntPtr, IntPtr)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(SByte, SByte)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(Single, Single)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(UInt16, UInt16)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(UInt32, UInt32)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(UInt64, UInt64)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write(UIntPtr, UIntPtr)	Writes the specified value to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.
Write<T>(T, T)	Writes the specified object reference to the specified field. On systems that require it, inserts a memory barrier that prevents the processor from reordering memory operations as follows: If a read or write appears before this method in the code, the processor cannot move it after this method.

See also

• volatile keyword (C# Reference)