Using "Lutz Roeder's .NET Reflector" on .NET Framwork shows that 
collections implement SyncRoot like this:

object ICollection.SyncRoot
{
    get
    {
        if (this._syncRoot == null)
        {
            Interlocked.CompareExchange(ref this._syncRoot, new object(), 
null);
        }
        return this._syncRoot;
    }
}


    Now I have to questions on this.

1) (major) Why is it made so? Why not initialize _syncRoot in the 
constructor, or even better in declaration like:

private object readonly _syncRoot = new object();

(Note that this also allows the use of readonly.) The construction by 
definition is executed only by on thread so no problem. And after the object 
is constructed the _syncRoot already exists so you do not have to check for 
its existance and do interlocked compare-exchange.
    Was optimalization the reason (no object created when no "call" to 
SyncRoot)? But on the other hand if you use SyncRoot you must pay for the 
null check and interlocked operation. And if one uses a collection of 
objects the one more empty object doesn't seem to do much difference. Or am 
I wrong here?

2) (minor) Why is this explicite implementation? (The code is extracted from 
System.Collections.Generic.LinkedList<T> but it seems to be exactly the same 
in every collection.)

    Thanks in advance!

    Adam Badura