How Do Lock Statements Work in C# to Ensure Thread Safety?

In-depth understanding of the thread safety mechanism of C# Lock statement

In a multi-threaded environment, it is crucial to ensure that objects are thread-safe. For this reason, developers often use lock statements to protect critical parts of the code. This article will delve into the inner workings of the lock statement, clarifying its behavior and impact.

Structure of Lock statement

Under the hood, the lock statement is converted into a series of instructions using the System.Threading.Monitor class. C# 3.0 adopts the following approach:

<code class="language-csharp">var temp = obj;
Monitor.Enter(temp);
try
{
    // 线程不安全代码
}
finally
{
    Monitor.Exit(temp);
}</code>

However, starting with C# 4.0, a small modification was introduced:

<code class="language-csharp">bool lockWasTaken = false;
var temp = obj;
try
{
    Monitor.Enter(temp, ref lockWasTaken);
    // 线程不安全代码
}
finally
{
    if (lockWasTaken)
    {
        Monitor.Exit(temp); 
    }
}</code>

Monitoring and Blocking

The

Monitor.Enter method is the core of the lock statement. It attempts to obtain the monitor for the specified object. If another thread has previously performed a Enter operation on the same object and has not yet performed a corresponding Exit operation, the current thread will block until the other thread releases the lock.

Waiting indefinitely

It is important to note that the Monitor.Enter method will wait indefinitely for the lock to become available. Unlike some other locking mechanisms, it has no concept of timeouts.

Performance Notes

Using locks can impact application performance. Locks introduce contention by temporarily preventing other threads from accessing the same resource. This can result in reduced throughput and increased latency. Therefore, it is important to use locks sparingly and only when necessary to protect thread-unsafe code.

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