为什么我会和QUOT;错误：...必须是引用类型"在我的C＃泛型方法？ [英] Why do I get "error: ... must be a reference type" in my C# generic method?

问题描述

在各种数据库中的表我都一个属性和一个值列。我使用LINQ到SQL来访问数据库

In various database tables I have both a property and a value column. I'm using Linq to SQL to access the database.

我正在写返回一个包含给定的数据库表中检索的属性/值的字典的方法：

I'm writing a method which returns a dictionary containing the properties/values retrieved from the given database table:

private static Dictionary<string, string> GetProperties<T>(Table<T> table)
{
    Dictionary<string, string> properties = new Dictionary<string, string>();

    foreach (var row in table)
    {
        properties[row.Property]=row.Value;
    }

    return properties;
}

一旦编译，我得到：

Upon compiling, I get:

错误1型'T'必须是引用类型，才能在泛型类型或方法使用它作为参数'TEntitySystem.Data这.Linq.Table< TEntity>

我试过没有运气搜索此错误消息。

I've tried searching for this error message without luck.

搜索计算器，这个问题似乎大同小异，但对于一个参数列表：的 http://stackoverflow.com/questions/1633690/generic-listt-as-parameter-on-method - 尽管参数仍然不在答案引用类型。对这个问题，无论是

Searching StackOverflow, this question seems similar, though regarding a parameter List: http://stackoverflow.com/questions/1633690/generic-listt-as-parameter-on-method - though the parameter still isn't a reference type in the answers to that question, either.

阅读MSDN上的C＃编程指南：的 http://msdn.microsoft.com/en-us/library/twcad0zb（VS.80）的.aspx 我看到他们的例子都按引用传递的参数。但是，我不能看到如何通过引用我的具体情况通过，因为泛型类型仅仅是用于指定泛型类型表。

Reading the C# Programming Guide on MSDN: http://msdn.microsoft.com/en-us/library/twcad0zb(VS.80).aspx I see their examples all pass the parameters by reference. However, I can't see how to pass by reference in my particular case, since the generic type is just for specifying the generic type of Table.

任何指针会。非常感谢

PS：如果Appologies需要时间，我接受一个答案，因为这个功能目前无法访问的（我是盲人和使用屏幕阅读器）

PS: Appologies if it takes time for me to accept an answer, as this feature is currently not accessible (I'm blind and use a screen reader).

推荐答案

这是因为如何表< T> 声明

public sealed class Table<TEntity> : IQueryable<TEntity>, 
    IQueryProvider, IEnumerable<TEntity>, ITable, IQueryable, IEnumerable, 
    IListSource
where TEntity : class  // <-- T must be a reference type!

编译器是抱怨，因为你的方法对 T没有约束，这意味着你可以接受 T 这不符合表1所述的规范; T>

The compiler is complaining because your method has no constraints on T, which means that you could accept a T which doesn't conform to the specification of Table<T>.

因此，你的方法需要至少与严格什么它接受。试试这个：

Thus, your method needs to be at least as strict about what it accepts. Try this instead:

private static Dictionary<string, string> GetProperties<T>(Table<T> table) where T : class

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