Entity Framework 实现 SQL “in”-优快云博客

本文介绍了一种在C#和VB.NET中构建LINQ查询的方法，该方法使用Lambda表达式来实现类似SQL IN子句的功能。通过传递选择器表达式和值集合，可以生成一个查询，用于检查实体属性是否包含在指定的值集中。

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

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

转载于:https://www.cnblogs.com/rock_chen/archive/2010/01/07/1641593.html