如何左右转换绑定表达式到更低？

Question

I am using below code to generate dynamic binding expression; here how to convert expression left and right part to lower? because I wanted to filter either lower case and upper case filter.

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

If i understand you correctly then you can use the following:

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));

In your switch replace member with left and constant with right

Update Regarding null checking
You can check if the member property is null by this: (let's say that originalExpression holds the expression from the switch )

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

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

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

Update 2
You should check for null values both on the property and on the value:

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);