Entity Framework 实现 SQL “in”

http://blogs.msdn.com/dsimmons/pages/ef-faq-query.aspx

C#

static Expression<Func<TElement, bool>> BuildContainsExpression<TElement, TValue>(

    Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values)

{

    if (null == valueSelector) { throw new ArgumentNullException("valueSelector"); }

    if (null == values) { throw new ArgumentNullException("values"); }

    ParameterExpression p = valueSelector.Parameters.Single();

    // p => valueSelector(p) == values[0] || valueSelector(p) == ...

    if (!values.Any())

    {

        return e => false;

    }

    var equals = values.Select(value => (Expression)Expression.Equal(valueSelector.Body, Expression.Constant(value, typeof(TValue))));

    var body = equals.Aggregate<Expression>((accumulate, equal) => Expression.Or(accumulate, equal));

    return Expression.Lambda<Func<TElement, bool>>(body, p);

}

var query2 = context.Entities.Where(

    BuildContainsExpression<Entity, int>(e => e.ID, ids));   

VB.NET

    Private Function BuildContainsExpression(Of TElement, TValue) _
        (ByVal valueSelector As Expressions.Expression(Of Func(Of TElement, TValue)), _
         ByVal values As IEnumerable(Of TValue)) _
          As Expressions.Expression(Of Func(Of TElement, Boolean))

        If valueSelector Is Nothing Then
            Throw New ArgumentNullException("valueSelector")
        End If

        If values Is Nothing Then
            Throw New ArgumentNullException("values")
        End If

        If Not values.Any() Then
            Return Function(n) False
        End If

        Dim equals = values.Select(Function(n) Expressions.Expression.Equal _
                                       (valueSelector.Body, Expressions.Expression.Constant(n, GetType(TValue))))

        Dim body = equals.Aggregate( _
        (Function(n1, n2) Expressions.Expression.Or(n1, n2)))

        Dim p As Expressions.ParameterExpression = valueSelector.Parameters.Single()

        Return Expressions.Expression.Lambda(Of Func(Of TElement, Boolean))(body, p)
    End Function