.NET Core中合并Expression<Func<T,bool>>的正确姿势

这是在昨天的 .NET Core 迁移中遇到的问题,之前在 .NET Framework 中是这样合并 Expression<Func<T,bool>> 的:

public static class ExpressionBuilder
{
    public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
    {
        var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
        var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
        return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
    }

    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.And);
    }
    
    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.Or);
    }
}

迁移至 .NET Core 之后,Entity Framework Core 生成了不正确的 SQL 语句:

WHERE (CASE
        WHEN ([q0].[IsActive] = 1) AND ([q.questionUser0].[IsActive] = 1)
        THEN CAST(1 AS BIT) ELSE CAST(0 AS BIT)
    END & CASE
        WHEN [q0].[DealFlag] = 0
        THEN CAST(1 AS BIT) ELSE CAST(0 AS BIT)
    END) AND (([q0].[Flags] & @__flags_0) = @__flags_1)

在 Server Server 中执行时会出现如下的错误:

在应使用条件的上下文(在 'AND' 附近)中指定了非布尔类型的表达式

后来参考 Combining two expressions (Expression<Func<T, bool>>),通过下面的代码解决了问题:

public static class ExpressionBuilder
{
    public static Expression<Func<T, bool>> And<T>(
        this Expression<Func<T, bool>> first, 
        Expression<Func<T, bool>> second)
    {
        return first.AndAlso<T>(second, Expression.AndAlso);
    }

    public static Expression<Func<T, bool>> Or<T>(
        this Expression<Func<T, bool>> first, 
        Expression<Func<T, bool>> second)
    {
        return first.AndAlso<T>(second, Expression.OrElse);
    }

    private static Expression<Func<T, bool>> AndAlso<T>(
    this Expression<Func<T, bool>> expr1,
    Expression<Func<T, bool>> expr2,
    Func<Expression, Expression, BinaryExpression> func)
    {
        var parameter = Expression.Parameter(typeof(T));

        var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
        var left = leftVisitor.Visit(expr1.Body);

        var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
        var right = rightVisitor.Visit(expr2.Body);

        return Expression.Lambda<Func<T, bool>>(
            func(left, right), parameter);
    }

    private class ReplaceExpressionVisitor
        : ExpressionVisitor
    {
        private readonly Expression _oldValue;
        private readonly Expression _newValue;

        public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
        {
            _oldValue = oldValue;
            _newValue = newValue;
        }

        public override Expression Visit(Expression node)
        {
            if (node == _oldValue)
                return _newValue;
            return base.Visit(node);
        }
    }

}
posted @ 2016-12-30 12:30 dudu 阅读(...) 评论(...) 编辑 收藏