泛型特化时，一般会遭遇无法用集合法来消除针对类型参数的 if 语句。通过使用匿名委托（需要静态泛型类作为 Holder），可以避免。其效果可以形容为作用于类型参数的模式匹配。如此也避免了可能存在的类型转换所带来的装箱/拆箱操作。

zrx-1-GenericTypeSpecialization.cs

void Main()
{
	IRecord record = new Record1();
	Console.WriteLine (record.Get<int>("aaa"));
}

public interface IRecord {
    string GetString(string field);
    int GetInt(string field);
    long GetLong(string field);
}

public class Record1 : IRecord{
	public string GetString(string field){return "1";}
    public int GetInt(string field) {return 1;}
    public long GetLong(string field) {return 1l;}
}

//public static class RecordExtensions {
//	public static T Get<T>(this IRecord record, string field) {
//		if(typeof(T) == typeof(string))
//        	return (T)(object)record.GetString(field);
//		if(typeof(T) == typeof(int))
//        	return (T)(object)record.GetString(field);
//		if(typeof(T) == typeof(long))
//        	return (T)(object)record.GetString(field);
//		return default(T);
//    }
//}

public static class RecordExtensions2 {
	private static class Cache<T>{
		public static Func<IRecord, string, T> Call;
	}
	
	static RecordExtensions2(){
		Cache<string>.Call = (record, field) => record.GetString(field);
		Cache<int>.Call = (record, field) => record.GetInt(field);
		Cache<long>.Call = (record, field) => record.GetLong(field);
	}

	public static T Get<T>(this IRecord record, string field) {
		if(Cache<T>.Call != null)
			return Cache<T>.Call(record, field);
		
		return default(T);
    }
}