在C＃中创建一个通用方法 [英] Creating a generic method in C#

问题描述

我试图将一堆类似的方法合并成一个通用的方法。我有几个返回查询字符串值的方法，如果查询字符串不存在或格式不正确，则返回null。如果所有类型都是本地可为空的，那么这很容易，但我必须使用可为空的泛型类型作为整数和日期。

I am trying to combine a bunch of similar methods into a generic method. I have several methods that return the value of a querystring, or null if that querystring does not exist or is not in the correct format. This would be easy enough if all the types were natively nullable, but I have to use the nullable generic type for integers and dates.

这是我现在拥有的。但是，如果数字值无效，它将返回0，并且不幸的是，在我的方案中是有效的值。有人可以帮我吗？

Here's what I have now. However, it will pass back a 0 if a numeric value is invalid, and that unfortunately is a valid value in my scenarios. Can somebody help me out? Thanks!

public static T GetQueryString<T>(string key) where T : IConvertible
{
    T result = default(T);

    if (String.IsNullOrEmpty(HttpContext.Current.Request.QueryString[key]) == false)
    {
        string value = HttpContext.Current.Request.QueryString[key];

        try
        {
            result = (T)Convert.ChangeType(value, typeof(T));  
        }
        catch
        {
            //Could not convert.  Pass back default value...
            result = default(T);
        }
    }

    return result;
}

推荐答案

如果您指定了默认值返回值，而不是使用默认值（T）？

What if you specified the default value to return, instead of using default(T)?

public static T GetQueryString<T>(string key, T defaultValue) {...}

它使调用变得更简单：

var intValue = GetQueryString("intParm", Int32.MinValue);
var strValue = GetQueryString("strParm", "");
var dtmValue = GetQueryString("dtmPatm", DateTime.Now); // eg use today's date if not specified

缺点是需要魔术值来表示无效/缺失查询字符串值。

The downside being you need magic values to denote invalid/missing querystring values.

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