最快的方法使用的DataContext从LINQ查询数据表填写 [英] Fastest way to fill DataTable from LINQ query using DataContext

问题描述

我试图运行一个LINQ查询，但我需要的结果作为一个数据表，因为我使用从不同的查询存储记录在同一个视图状态对象。

I am trying to run a linq query but I need the result as a datatable as I am using that to store records from different queries in the same viewstate object.

2.以下版本编译，但返回一个空集。确切的错误是的值不能为空。
参数名：源的（是的，我已经检查有数据）：

The 2 versions below compile, but return an empty set. The exact error is "Value cannot be null. Parameter name: source". (and yes I have checked there is data):

MyDatabaseDataContext db = new MyDatabaseDataContext(conn); 
IEnumerable<DataRow> queryProjects = 
    (from DataRow p in db.STREAM_PROJECTs.AsEnumerable()
    where p.Field<int>("STREAM_ID") == StreamID
    select new
    {
        PROJECT_ID = p.Field<int>("PROJECT_ID"),
        PROJECT_NAME = p.Field<string>("PROJECT_NAME")
    }) as IEnumerable<DataRow>;
DataTable results = queryProjects.CopyToDataTable<DataRow>();

...

//(from p in db.STREAM_PROJECTs.AsEnumerable()
//where p.STREAM_ID == StreamID
//select new
//{
//    p.PROJECT_NAME,
//    p.PROJECT_ID
//}) as IEnumerable<DataRow>;

在此<一个例子href=\"http://stackoverflow.com/questions/16/how-do-i-fill-a-dataset-or-a-datatable-from-a-linq-query-resultset\">thread似乎并没有在这种情况下工作无论是。

The examples in this thread don't seem to work in this situation either.

我想我可以只运行SQL查询命令老式的方法，但不LINQ应该是更快？

I guess I could just run a sql query command the old-fashioned way, but isn't linq supposed to be quicker?

推荐答案

您的问题是这样的：

as IEnumerable<DataRow>

的为关键字进行安全投，而不是一个转换，这好像你可能会认为它在做什么。在为关键词语义相同，这样做的：

The as keyword performs a safe cast, not a conversion, which it seems like you might think that it's doing. The as keyword is semantically the same as doing this:

IEnumerable<DataRow> queryProjects = 
    (IEnumerable<DataRow>)(from DataRow p in db.STREAM_PROJECTs.AsEnumerable()
    where p.Field<int>("STREAM_ID") == StreamID
    select new
    {
        PROJECT_ID = p.Field<int>("PROJECT_ID"),
        PROJECT_NAME = p.Field<int>("PROJECT_NAME")
    });

除非的版本时，它不能投你的查询对象（这是一个的IQueryable＆LT不会抛出异常; T＆GT; ，其中 T 是一个匿名类型）到的IEnumerable＆LT; D​​ataRow的＆GT; （它ISN 'T）。

Except the version with as won't throw an exception when it fails to cast your query object (which is an IQueryable<T>, where T is an anonymous type) to an IEnumerable<DataRow> (which it isn't).

不幸的是，没有内置的方法，我所知道的，将采取具体类型（如在本例中您的匿名类型）的枚举，并把它变成一个数据表。写作人会不会太复杂，因为你基本上是需要反思地获取属性，然后遍历集合，并使用这些属性在数据表以创建列。我会在几分钟后一个例子。

Unfortunately, there is no built-in method that I'm aware of that will take an enumerable of a concrete type (like your anonymous type in this example) and turn it into a DataTable. Writing one wouldn't be too complicated, as you'd essentially need to get the properties reflectively then iterate over the collection and use those properties to create columns in a DataTable. I'll post an example in a few.

这样的事情，放在一个静态类你使用的在命名空间中，应该提供一个扩展方法，将你想要做什么：

Something like this, placed in a static class within a namespace that you're using, should provide an extension method that will do what you want:

public static DataTable ToDataTable<T>(this IEnumerable<T> source)
{
    PropertyInfo[] properties = typeof(T).GetProperties();

    DataTable output = new DataTable();

    foreach(var prop in properties)
    {
        output.Columns.Add(prop.Name, prop.PropertyType);
    }

    foreach(var item in source)
    {
        DataRow row = output.NewRow();

        foreach(var prop in properties)
        {
            row[prop.Name] = prop.GetValue(item, null);
        }

        output.Rows.Add(row);
    }

    return output;
}

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