c# - 如何创建动态 Lambda 表达式

Question

我有一个问题，如何添加另一个过滤器并且我必须验证它是否已被选中？

private Expression < Func < Entity.Modelos.Flux, bool >> Filter() {
    var dateStart = dtpDateStart.Value.Date;
    var dateEnd = dtpDateEnd.Value.Date;

    Expression < Func < Entity.Modelos.Flux, bool >> expr = null;

    expr = f = > f.DatFlux >= dateStart.Date && f.DatFlux <= dateEnd.Date;

    if (txtDescription.Text != String.Empty) {
        //add filter
    }

    return expr;
}

更新:我将在此函数中使用表达式:

public virtual IQueryable < T > Filter(Expression < Func < T, bool >> expressao) {
    return DbSet.Where(expressao).AsQueryable < T > ();
}

我想做的是这个，但是有一个表达式

public List < Users > GetUsers(int ? id, string name) {
    using(DBContext ctx = new DBContext()) {
        IQueryable query = ctx.Usuarios;
        if (id.HasValue)
            query = query.Where(x = > x.ID == id);

        if (!string.IsNullOrEmpty(name))
            query = query.Where(x = > x.Name.StartsWith(name));

        return query.ToList();

    }
}

Answer 1

继承自ExpressionVisitor

public class MyVisitor: ExpressionVisitor
{
    private LambdaExpression visitor;
    public Expression Modify(Expression expression, LambdaExpression visitor)
    {
        this.visitor = visitor;
        return Visit(expression);
    }

    protected override Expression VisitBinary(BinaryExpression b)
    {
        var binary = visitor.Body as BinaryExpression;

        return Expression.MakeBinary(ExpressionType.AndAlso, b, binary, b.IsLiftedToNull, b.Method);
    }
}

您的 Filter() 方法可能如下所示

    private Expression<Func<Entity.Modelos.Flux, bool>> Filter()
    {
        var dateStart = dtpDateStart.Value.Date;
        var dateEnd = dtpDateEnd.Value.Date;
        var description = txtDescription.Text;

        Expression<Func<Entity.Modelos.Flux, bool>> expr = null;

        expr = f => f.DatFlux >= dateStart.Date && f.DatFlux <= dateEnd.Date;

        if (description != String.Empty)
        {
            //add filter
            Expression<Func<Entity.Modelos.Flux, bool>> other = f => f.Description == description;

            var modifier = new MyVisitor();
            expr = (Expression<Func<Entity.Modelos.Flux, bool>>)modifier.Modify((Expression)expr, (LambdaExpression)other);
        }

        return expr;
    }

example

请查看以下内容以获取更多信息

How to: Modify Expression Trees (C# and Visual Basic)