这两种使用foreach的方式有区别吗？

Question

Is there a difference between the two code snippets shown below?

1. Property loaded into variable before the foreach clause.

    var someCollection = someClass.Collection; foreach (var elem in someCollection) { // Do something } 

2. Property accessed inline within the foreach clause.

    foreach (var elem in someClass.Collection) { // Do something } 

Is second variant less optimized and if so, why?

Answer 1

There is no difference between the two approaches, optimization- or performance-wise. To prove this, I have created a brief example similar to yours:

public class C {
    public void A() {
        foreach (var item in SomeClass.Collection) {}
    }

    public void B() {
        var someCollection = SomeClass.Collection;
        foreach (var item in someCollection) {}
    }   
}

public static class SomeClass {
    public static List<string> Collection { get; set; }
}

When compiled using the Roslyn .NET compiler version 2.9.0, both methods A and B result in the very same intermediate code as shown below ( reference ).

Method Body of A and B

List<string>.Enumerator enumerator = SomeClass.Collection.GetEnumerator();
try
{
    while (enumerator.MoveNext())
    {
        string current = enumerator.Current;
    }
}
finally
{
    ((IDisposable)enumerator).Dispose();
}

Answer 2

There is no difference in performance or optimization. The only difference is that you have one more variable in your code, which holds a reference (an address, a "pointer") to your collection.