作为方法参数的连接谓词的默认值

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问题描述

我有一个选择数据的方法.如果调用者可以提供一个谓词来修改 .Where()，它可以支持多个用例.我尝试过类似

private class ABJoin
{
    public A A { get; set; }
    public B B { get; set; }
}

bool NoFilter(ABJoin join, int index)
{
    return true; // Don't filter at all for this
}

private IEnumerable<TResult> GetData
(Func<ABJoin, int, bool> filter)
{
    var query = ctx.TypeA
        .Join(ctx.TypeB, a => a.BId, b => b.Id, 
              (a, b) => new ABJoin() { A = a, B = b })
    // etc.
}

到目前为止效果很好.

但是，有些用例不需要提供任何过滤器(实际版本有其他参数来区分每个用例的行为).我认为为过滤器参数提供默认值会很方便

private IEnumerable<TResult> GetData
(Func<ABJoin, int, bool> filter = NoFilter)

但是，这不会编译.错误指出 NoFilter 必须是编译时常量.

有没有办法为 GetData() 中的 filter 提供默认值?

推荐答案

提供 null 的默认值，并在方法内将其与真正的委托交换:

private IEnumerable<TResult> GetData(Func<ABJoin, int, bool> filter = null)
{
    filter = filter ?? ((a,b) => true);
}

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问题描述

I have a method that selects data. It can support multiple use cases if the caller can provide a predicate to modify the .Where(). I attempted something like

private class ABJoin
{
    public A A { get; set; }
    public B B { get; set; }
}

bool NoFilter(ABJoin join, int index)
{
    return true; // Don't filter at all for this
}

private IEnumerable<TResult> GetData
(Func<ABJoin, int, bool> filter)
{
    var query = ctx.TypeA
        .Join(ctx.TypeB, a => a.BId, b => b.Id, 
              (a, b) => new ABJoin() { A = a, B = b })
    // etc.
}

Works great, so far.

However, some use cases do not need to provide any filter (the real version has other parameters to distinguish behavior per use case). I thought it would be handy to provide a default value for the filter parameter

private IEnumerable<TResult> GetData
(Func<ABJoin, int, bool> filter = NoFilter)

However, that does not compile. The error states that NoFilter must be a compile-time constant.

Is there a way to provide a default value for filter in GetData()?

推荐答案

Provide default value of null and swap it with real delegate inside the method:

private IEnumerable<TResult> GetData(Func<ABJoin, int, bool> filter = null)
{
    filter = filter ?? ((a,b) => true);
}

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