Dynamic Linq/Lambda Filtering

Question

I havea method that returns Expression to be used for dynamic filtering of records based on the clients needs, i have a problem in doing it, i want something like this

     public Expression<T> FilterCreator<T>(FilterCondition condition, string columnName, object value)
    {

        Expression<Func<Customer, bool>> query;

        // FilterCondition is an enum flag for conditions
        if(condition.Condition == ConditionFlags.EQUALS)
        {
            // here is the problem,
            // i want the emailAddress to be dynamic based on the passed columName parameter of the client 
            // and be able to cast its type of the value that was passed
            query = p => p.EmailAddress == (typeof(p.EmailAddress))value;

           //i want something like this
           // query = p => p.(columnName)=> (typeOf(p.(columnName)))value;

        }
        else if(condition.Condition == ConditionFlags.CONTAINS)
        {
             .....
        } 

        return query;

    }

any advise guys? thanks in advance

SLaks · Accepted Answer · 2011-05-12 11:50:10Z

2

You need to build an expression tree:

var param = Expression.Parameter<Customer>();
p = Expression.LambdaFunc<Customer, bool>(
    Expression.Call(typeof(object), "Equals", null, //non-generic
                    Expression.Property(param, columnName),
                    Expresssion.Constant(value)
    )
);

edited May 12, 2011 at 11:50

answered May 12, 2011 at 11:35

SLaks

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4 Comments

dotnetlinc Over a year ago

where is the == operator on that tree? how about expression for the .Contains method of a string.

SLaks Over a year ago

@dot: Nowhere; It calls Object.Equals instead. This makes string equality work correctly (comparing by value). You can also build expression trees that call other methods; see the documentation.

dotnetlinc Over a year ago

uh, i get it, so when i want to use Contains(), ill just call typeof(string) and call the method "Contains" ? thanks

SLaks Over a year ago

No; Contains is an instance method, so you call it on an instance rather than a type. Expression.Call(Expression.Property(param, columnName), "Contains", null, Expresssion.Constant(value))

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