C＃：专业模板方法 - 错误：类型'...'已经定义了一个叫'...'具有相同的参数类型成员 [英] C#: specialized template method - Error: Type '...' already defines a member called '...' with the same parameter types

问题描述

我很新的C＃和目前正在开发使用的EntityFramework的应用程序。我想扩展数据库上下文类的功能，这样我就可以调用方法getPool（），以便它捧出来的类的根据DbSet成员。

I'm quite new to C# and currently developing an application using the EntityFramework. I would like to extend the functionality of the database context class, so that I can call a method getPool() so that it hands out the according DbSet member of the class.

我要实现它作为一个模板，因为它会从其他模板，这是只知道对（全局）数据库上下文对象和一个T型（具有给定超类）为他们应查询数据库，供以后调用。

I need to implement it as a template as it will be later called from other templates, which are just knowing about the (global) database context object and a type T (having a given superclass) for which they shall query the database.

下面是我试了一下（有点简单化 - 原来的例子太复杂了）：

Here is what I tried (a bit simplified - original example is too complicated):

public class TestContext : DbContext
{
    public DbSet<TestA> ATests { get; set; }
    public DbSet<TestB> BTests { get; set; }

    public IQueryable<T> getPool<T>() where T : TestA {
       return (IQueryable<T>)ATests;
    }
    public IQueryable<T> getPool<T>() where T : TestB {
       return (IQueryable<T>)BTests;
    }
}

该错误消息

The error message is

错误：类型'...'已经定义了一个叫'...'具有相同的参数类型

和它发生在我的模板的第二个专门定义的线（公开的IQueryable< T> getPool< T>（）其中T：TESTB ）

And it occurs at the line of the second specialized definition of my template (public IQueryable<T> getPool<T>() where T : TestB).

现在的问题是：如何来解决这个问题。

The question is: How to fix this?

推荐答案

不幸的是，在C＃中，您不能使用泛型类型约束这样的重载方法。你必须给他们这样的不同的名字。

Unfortunately, in C#, you cannot overload a method by using a generic type constraint like this. You will have to give them different names like this

public class TestContext : DbContext
{
    public DbSet<TestA> ATests { get; set; }
    public DbSet<TestB> BTests { get; set; }

    public IQueryable<T> getPoolA<T>() where T : TestA {
       return (IQueryable<T>)ATests;
    }
    public IQueryable<T> getPoolB<T>() where T : TestB {
       return (IQueryable<T>)BTests;
    }
}

另一种解决办法是做这样的事情：

Another solution would be to do something like this:

public class TestContext : DbContext
{
    public DbSet<TestA> ATests { get; set; }
    public DbSet<TestB> BTests { get; set; }

    public IQueryable<T> getPool<T>() {
       return (typeof(T) == typeof(TestA))
                  ? (IQueryable<T>)ATests
                  : (IQueryable<T>)BTests;
    }
}

现在，你甚至可以清洁做这一点，因为< A HREF =htt​​p://msdn.microsoft.com/en-us/library/bb351562.aspx相对=nofollow> 的IQueryable< T> 在 T 是协变，你能避免铸造：

Now, you can make this even cleaner, since IQueryable<T> is covariant in T, you can avoid casting:

public class TestContext : DbContext
{
    public DbSet<TestA> ATests { get; set; }
    public DbSet<TestB> BTests { get; set; }

    public IQueryable<T> getPool<T>() {
       return (typeof(T) == typeof(TestA)) ? ATests : BTests;
    }
}

---

如果你想避免类型的测试，你可以做这样的事情：

---

If you want to avoid testing for types you can do something like this:

public class TestContext : DbContext
{
    readonly Dictionary<Type, object> _sets;

    public DbSet<TestA> ATests { get; set; }
    public DbSet<TestB> BTests { get; set; }

    public TestContext()
    {
        _sets = new Dictionary<Type, object>
        {
            { typeof(TestA), ATests },
            { typeof(TestB), BTests }
        }
    }

    public IQueryable<T> getPool<T>() {
       return (IQueryable<T>)_sets[typeof(T)];
    }
}

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