How to Use the Contains() Function in Linq to Entities?-C++-php.cn

How to Use the Contains() Function in Linq to Entities?

Susan Sarandon

Release： 2024-10-29 11:14:29

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How to Use the Contains() Function in Linq to Entities?

Contains() Function Workaround Using Linq to Entities

Question:

How to use the Contains() function within Linq to Entities queries, despite its lack of support?

Answer:

Update: As of Entity Framework (EF) version 4 and later, Contains() is supported directly.

For earlier versions of EF, a workaround is necessary:

Custom Extension Method for Contains()

Create the following extension method:

<code class="csharp">public static IQueryable<TEntity> WhereIn<TEntity, TValue>(
    this ObjectQuery<TEntity> query,
    Expression<Func<TEntity, TValue>> selector,
    IEnumerable<TValue> collection)
{
    // ... (implementation remains the same as in the provided code)
}</code>

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Optional Static Collection Version

Optionally, you can create a version that allows a static collection as input:

<code class="csharp">public static IQueryable<TEntity> WhereIn<TEntity, TValue>(
    this ObjectQuery<TEntity> query,
    Expression<Func<TEntity, TValue>> selector,
    params TValue[] collection)
{
    return WhereIn(query, selector, (IEnumerable<TValue>)collection);
}</code>

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Usage:

With this extension method, you can use Contains() in your Linq to Entities queries as follows:

<code class="csharp">public static void Main()
{
    using (MyObjectContext context = new MyObjectContext())
    {
        // Using method 1 - collection provided as collection
        var contacts1 = context.Contacts.WhereIn(c => c.Name, GetContactNames());

        // Using method 2 - collection provided statically
        var contacts2 = context.Contacts.WhereIn(c => c.Name,
            "Contact1",
            "Contact2",
            "Contact3",
            "Contact4");
    }
}</code>

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