强制从DataReader的只返回一行 [英] Enforce only single row returned from DataReader

问题描述

我似乎在我的code写了不少：

I seem to write this quite a lot in my code:

using (var reader = cmd.ExecuteReader())
{
    if (reader.Read())
    {
        result = new User((int)reader["UserId"], reader["UserName"].ToString());
    }

    if (reader.Read())
    {
        throw new DataException("multiple rows returned from query");
    }
}

有一些内置的方式做到这一点，我不知道？

Is there some built in way to do this that I don't know about?

推荐答案

我不知道，但是这code可以被委派到一个扩展方法：

I don't know, but this code can be delegated into an extension method:

public static R Single<R>(this DataReader reader, Func<DataReader,R> selector) {
    R result = default(R);
    if (reader.Read())
        result = selector(reader);
    if (reader.Read())
        throw new DataException("multiple rows returned from query");
    return result;
}

要使用这样的：

using (var reader = cmd.ExecuteReader())
{
    User u = reader.Single(r => new User((int)r["UserId"], r["UserName"].ToString()))
}

节省您从code复制。

Saving you from code duplication.

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