Using a LINQ Where query to get only some of the ConfigurationManager.ConnectionStrings

问题

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).

I started with this, which does not work:

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

It tells me that:

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.

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!");
}

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

Could not resolve symbol 'Name'

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!");
}

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

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?

回答1:

TL;DR Version

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

Details

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);

This is tells you:

Cannot convert instance argument type 'System.Configuration.ConnectionStringSettingsCollection' to 'System.Collections.Generic.IEnumerable<System.Configuration.ConnectionStringSettings>'

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.

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"));

Enjoy!

来源：https://stackoverflow.com/questions/26419655/using-a-linq-where-query-to-get-only-some-of-the-configurationmanager-connection