linq选择字段类型未知

Question

I have the following code:

var query =
        from product in products.AsEnumerable()
        where product.Field<string>("Product") == "Phone"
        select new
        {
            Name = product.Field<string>("Name"),
            ProductNumber = product.Field<string>("ProductNumber"),
            ListPrice = product.Field<Decimal>("Price")
        };

But I am getting the following error:

Unable to cast object of type 'System.Int32' to type 'System.String'.

I assume that's because in the ProductNumber column I don't always have doesn't always have strings, and in the first row the ProductNumber is actually an int . I've tried converting and casting them to string, but didn't work.

How can I fix this?

Answer 1

From the documentation of Field<T> :

InvalidCastException [is thrown when] the value type of the underlying column could not be cast to the type specified by the generic parameter, T.

It looks like in your case the column type is a nullable integer, so you need to do something like this:

var query =
    from product in products.AsEnumerable()
    where product.Field<string>("Product") == "Phone"
    select new
    {
        Name = product.Field<string>("Name"),
        // Below, I am using ""+... idiom for a null-safe ToString
        ProductNumber = ""+product.Field<int?>("ProductNumber"),
        ListPrice = product.Field<Decimal>("Price")
    };

The idea is to retrieve the value as an int , but accept data rows where the data is missing.

If you do not mind ProductNumber of your anonymous class being nullable int s instead of string s, remove the ""+ part of the expression.

Answer 2

首先，使用ProductNumber作为字符串获取可枚举的记录，然后在Enumerable上进行从字符串到整数的转换。