c# - 如何将Binding Expression left 和 right 转换为lower？

Question

我正在使用下面的代码生成动态绑定(bind)表达式；这里如何将表达式的左右部分转换为较低的？因为我想过滤小写和大写过滤器。

private static Expression GetExpression<T>(ParameterExpression param, Filter filter)
    {
        MemberExpression member = Expression.Property(param, filter.PropertyName);
        ConstantExpression constant = Expression.Constant(filter.Value);

        switch (filter.Operation)
        {
            case Op.Equals:
                return Expression.Equal(member, constant);

            case Op.GreaterThan:
                return Expression.GreaterThan(member, constant);

            case Op.GreaterThanOrEqual:
                return Expression.GreaterThanOrEqual(member, constant);

            case Op.LessThan:
                return Expression.LessThan(member, constant);

            case Op.LessThanOrEqual:
                return Expression.LessThanOrEqual(member, constant);

            case Op.Contains:
                return Expression.Call(member, containsMethod, constant);

            case Op.StartsWith:
                return Expression.Call(member, startsWithMethod, constant);

            case Op.EndsWith:
                return Expression.Call(member, endsWithMethod, constant);
        }

        return null;
    }

Answer 1

如果我理解正确，那么您可以使用以下内容:

MemberExpression member = Expression.Property(param, filter.PropertyName);
ConstantExpression constant = Expression.Constant(filter.Value);

Expression left = Expression.Call(member, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
Expression right = Expression.Call(constant, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));

在您的开关中，将 member 替换为 left 并将 constant 替换为 right

关于空检查的更新
您可以通过以下方式检查成员属性是否为空:(假设 originalExpression 包含来自 switch 的表达式)

var nullCheck = Expression.Equal(member, Expression.Constant(null, member.Type));

Expression.IfThenElse(nullCheck, Expression.Constant(false), originalExpression);

Expression.Conditional(nullCheck, Expression.Constant(false), originalExpression);

更新 2
您应该检查属性和值上的空值:

var nullCheckProp = Expression.Equal(member, Expression.Constant(null, member.Type)); 
var nullCheckVal = Expression.Equal(constant, Expression.Constant(null, constant.Type)); 
var nullCheckBoth = Expression.OrElse(nullCheckProp,nullCheckVal);

Expression.Conditional(nullCheckBoth, Expression.Constant(false), originalExpression);