Question

我想将谓词构建逻辑移动到基类中，并且得到错误“LINQ表达式节点类型＆＃39;调用＆＃39; LINQ to Entities不支持“。我希望能够在谓词中连接或有条件地链接表达式。

我希望能够传入谓词的一部分，该谓词对于调用者来说是唯一的属性名称（ GetFilterPredicate 将成为一个通用例程，它将对将要使用的类型进行操作有不同的属性名称持有相关的值）。

        protected Expression<Func<MyEntity, bool>> GetFilterPredicate(
        PagingParameters pagingParameters,
        Func<MyEntity, DateTime?> terminationDate,
        Func<MyEntity, string> entityID,
        Func<MyEntity, string> name
        )
    {
        Expression<Func<MyEntity, bool>> validFilter = x => true;

        if (pagingParameters.Active == true)
        {
            validFilter = x => terminationDate(x) > DateTime.Now;
        }

        ///more conditions added here

        return validFilter;
    }

    protected List<MyEntity> Query(IQueryable<MyEntity> source)
    {
        var filters = GetFilterPredicate(
            new PagingParameters() { Active = true }
            , i => i.TerminationDate
            , i => i.EntityID
            , i => i.Name
            );

        return source.Where(filters).AsNoTracking().ToList<MyEntity>();
    }

Answer 1

您无法使用委托terminationDate构建新表达式，您需要将terminationDate更改为表达式并使用它来手动构建新表达式。

protected static  Expression<Func<MyEntity, bool>> GetFilterPredicate(
    PagingParameters pagingParameters,
    Expression<Func<MyEntity, DateTime?>> terminationDate,
    Expression<Func<MyEntity, string>> entityID,
    Expression<Func<MyEntity, string>> name
)
{
    Expression<Func<MyEntity, bool>> validFilter = x => true;
    // We need to replace the parameter for all expressions with 
    // a common single parameter. I used the parameter for the default
    // filter but a new Parameter expression would have worked as well.
    // If you don't do this you will get an error (unbound parameter or something like that ) because the parameter  
    // used in the expressions (terminationDate, entityID) will be 
    // different then the parameter used for the new validFilter expression
    var parameterReplacer = new ReplaceVisitor
    {
        NewParameter = validFilter.Parameters.First()
    };

    if (pagingParameters.Active == true)
    {
        validFilter = Expression.Lambda<Func<MyEntity, bool>>(
            Expression.GreaterThan
            (
                parameterReplacer.Visit(terminationDate.Body),
                Expression.Convert(Expression.Property(null, typeof(DateTime), "Now"), typeof(DateTime?))
            ),
            parameterReplacer.NewParameter
        );
    }

    // existing filter && x.EntityId != "A"
    validFilter = Expression.Lambda<Func<MyEntity, bool>>(
            Expression.And(
                validFilter.Body,
                Expression.NotEqual
                (
                    parameterReplacer.Visit(entityID.Body),
                    Expression.Constant("A")
                )
            ),
            parameterReplacer.NewParameter
        );

    return validFilter;
}
/// <summary>
/// Simple Parameter Replacer, will replace the any parameter with the new
/// parameter. You will need to refine this if your expressions have nested 
/// lambda, in that you will need to only replace the top most lambda 
/// parameter, but for simple expressions it will work fine.
/// </summary>
class ReplaceVisitor : ExpressionVisitor
{
    public ParameterExpression NewParameter { get; set; }
    protected override Expression VisitParameter(ParameterExpression node)
    {
        return this.NewParameter;
    }
}

Answer 2

您可以手动执行此操作，如Titian答案所述，但更简单的方法是使用LinqKit。首先安装LinqKit nuget包并添加Invoke，然后：

Expand

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创建的谓词在Linq to Entity Framework Where子句中不起作用

2 个答案: