另一个技巧将通过访问InternalIndexOfHashSet的内部函数来进行反射。请记住，字段名是硬编码的，因此，如果在即将发布的.NET版本中更改了字段名，则该字段名将被破坏。
注意：如果使用Mono，则应将字段名称从更改m_slots为_slots。
internal static class HashSetExtensions<T>
{
public delegate bool GetValue(HashSet<T> source, T equalValue, out T actualValue);
public static GetValue TryGetValue { get; }
static HashSetExtensions() {
var targetExp = Expression.Parameter(typeof(HashSet<T>), "target");
var itemExp = Expression.Parameter(typeof(T), "item");
var actualValueExp = Expression.Parameter(typeof(T).MakeByRefType(), "actualValueExp");
var indexVar = Expression.Variable(typeof(int), "index");
var indexExp = Expression.Call(targetExp, typeof(HashSet<T>).GetMethod("InternalIndexOf", BindingFlags.NonPublic | BindingFlags.Instance), itemExp);
var truePart = Expression.Block(
Expression.Assign(
actualValueExp, Expression.Field(
Expression.ArrayAccess(
Expression.Field(targetExp, typeof(HashSet<T>).GetField("m_slots", BindingFlags.NonPublic | BindingFlags.Instance)), indexVar),
"value")),
Expression.Constant(true));
var falsePart = Expression.Constant(false);
var block = Expression.Block(
new[] { indexVar },
Expression.Assign(indexVar, indexExp),
Expression.Condition(
Expression.GreaterThanOrEqual(indexVar, Expression.Constant(0)),
truePart,
falsePart));
TryGetValue = Expression.Lambda<GetValue>(block, targetExp, itemExp, actualValueExp).Compile();
}
}
public static class Extensions
{
public static bool TryGetValue2<T>(this HashSet<T> source, T equalValue, out T actualValue) {
if (source.Count > 0) {
if (HashSetExtensions<T>.TryGetValue(source, equalValue, out actualValue)) {
return true;
}
}
actualValue = default;
return false;
}
}
测试：
var x = new HashSet<int> { 1, 2, 3 };
if (x.TryGetValue2(1, out var value)) {
Console.WriteLine(value);
}