Why Does C#'s `foreach` Loop Variable Reuse Cause Problems with Closures?

C#

Reuse of cycle variables: an unnecessary trap foreach When using lambda expression or anonymous method in C#, a modified closure access may appear.

The reuse of variables in the cycle has exacerbated this problem.

foreach The compiler usually declares the cycle variable outside the cycle body, as shown in the modified closure example:

This is different from the expectations of declared variables in the cycle:

string s;
while (enumerator.MoveNext())
{
   s = enumerator.Current;
   ...
}

The external variable statement has exacerbated the modified closure access problem because it creates a longer scope for variables. The variables declared in the <环> cycle cannot be accessed outside the cycle.

while (enumerator.MoveNext())
{
   string s;
   s = enumerator.Current;
   ...
}

The compiler decides to reuse the circular variable before the introduction of Lambda expression and anonymous method. Therefore, there was no potential trap that the variable reuse was fully considered at the time. foreach

In C# 5, this design defect will be resolved through a major change. The circular variable will be logically inside the cycle body to ensure that each closure gets a new copy. This change eliminates the closure trap of the cycle modified.

Although this repair is a popular improvement, when using the old version of the old version or not supporting this repair, it still needs to pay attention to this problem.

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