使用LINQ Where查询仅获取一些ConfigurationManager.ConnectionStrings [英] Using a LINQ Where query to get only some of the ConfigurationManager.ConnectionStrings

问题描述

我的目标是在控制台应用程序中的ConfigurationManager.ConnectionStrings集合上使用LINQ Where查询(假设添加了System.Configuration引用的新.NET 4.5控制台应用程序和相应的using语句).

My goal is to use a LINQ Where query on the ConfigurationManager.ConnectionStrings collection in a console application (assume a fresh .NET 4.5 console application with a System.Configuration reference added, and a correspodning using statement).

我从这里开始，它无效有效:

I started with this, which does not work:

var relevantSettings =
        ConfigurationManager.ConnectionStrings.Where(s => s.Name.StartsWith("Xyz"));

它告诉我:

无法从用法中推断出方法'IEnumerable<TSource> System.Linq.Enumerable.Where<TSource(this IEnumerable<TSource>, Func<TSource,bool>)'的类型参数.尝试显式指定参数.

The type arguments for method 'IEnumerable<TSource> System.Linq.Enumerable.Where<TSource(this IEnumerable<TSource>, Func<TSource,bool>)' cannot be inferred from the usage. Try specifying the arguments explicitly.

这使我措手不及，所以我尝试这样做来检查自己的理智，但是这不有效:

This caught me off guard, so I tried this to check my sanity, but this does not work either:

foreach (var settingsin ConfigurationManager.ConnectionStrings)
{
    if (settings.Name.StartsWith("Xyz")) Console.WriteLine("Found one!");
}

它抱怨不是 关于foreach语句，而是关于.Name位，并且报错:

It complains not about the foreach statement, but about the .Name bit, with the error:

无法解析符号名称"

Could not resolve symbol 'Name'

我了解到，可能有某种阻止编译器推断var类型的方法，请仔细检查我是否尝试了可行的这种方法:

I understand that there's probably something preventing the compiler from inferring the var's type, to double check I tried this which does work:

foreach (ConnectionStringSettings settings in ConfigurationManager.ConnectionStrings)
{
    if (connectionString.Name.StartsWith("Xyz")) Console.WriteLine("Found one!");
}

但是，除了解决我眼前的问题之外，这对我没有太大帮助.我想了解这是怎么回事.

However, that doesn't help me very much, except for solving my immediate problem. I want to understand what is going on here.

我要做的就是使用简单的LINQ Where语句在我的app.config中获取连接字符串的子集.为什么编译器阻止我这样做?

All I wanted to do is use a simple LINQ Where statement to get a subset of the connection strings in my app.config. Why is the compiler preventing me from doing this?

推荐答案

TL; DR版本

您需要执行.Cast<ConnectionStringSettings>()才能完成此工作.

TL;DR Version

You need to do a .Cast<ConnectionStringSettings>() to make this work.

好吧，如果您进行更深入的研究，您可以让编译器告诉您更多信息:

Well, if you dig even a little deeper, you can get the compiler to tell you even more:

Func<ConnectionStringSettings, bool> StartsWithXyz = c => c.Name.StartsWith("Xyz");
var relevantSettings = ConfigurationManager.ConnectionStrings.Where(StartsWithXyz);

这是告诉你的:

无法将实例参数类型'System.Configuration.ConnectionStringSettingsCollection'转换为'System.Collections.Generic.IEnumerable<System.Configuration.ConnectionStringSettings>'

从ConnectionStringSettings属性的继承树开始，您最终可以看到罪魁祸首，这很有道理:该属性属于 non 通用IEnumerable类型.

Driving up the inheritance tree of the ConnectionStringSettings property you can finally see the culprit, which makes a lot of sense: that property is of the non generic IEnumerable type.

实际上，这使该问题与此问题相当复杂，因为解决方案在于投射非将通用属性更改为通用列表，以便LINQ和编译器可以发挥作用.对于这种特定情况，可以执行以下 :

This in fact makes the question a rather elaborate duplicate of this question, as the solution lies in casting the non-generic property to a generic list, so that LINQ and the compiler may do their magic. For this particular scenario, the following does work:

var relevantSettings = ConfigurationManager.ConnectionStrings
                                           .Cast<ConnectionStringSettings>()
                                           .Where(c => c.Name.StartsWith("Xyz"));

享受！

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