List<testObject> objectList = new List<testObject>();

// populate objectList with testObjects

objectList.FindAll(GroupLevel0);

private static bool GroupLevel0(testObject item)
{ return item._groupLevel == 0; }

private class testObject
{
     public string _FieldSQL = null;
     public int _groupLevel;
}

List<testObject> objectList = new List<testObject>();
int desiredGroupLevel = 10;

objectList.FindAll(delegate (testObject item)
{
    return item._groupLevel == desiredGroupLevel;
});

List<testObject> objectList = new List<testObject>();
int desiredGroupLevel = 10;

objectList.FindAll(CheckGroupLevel(desiredGroupLevel));

...

public Predicate<testItem> CheckGroupLevel(int level)
{
    return delegate (testItem item)
    {
        return item._groupLevel == level;
    };
}

int groupLevel = 0;

  objectList.FindAll(
       delegate(testObject item)
       {
          return item._groupLevel == groupLevel;
       });

List<testObject> objectList = new List<testObject>();

// populate objectList with testObjects

objectList.FindAll(delegate(testObject o){ return GroupLevel(o, 0);} );

private static bool GroupLevel(testObject item, int groupLevel)
{ return item._groupLevel == groupLevel; }