Need help to build Expression Tree

Hi Experts,

My Name is Somnath .I am working ASP.net Project in which I am using Linq

I am creating Dynamic Lamabada Expression using Expression tree

I want to use DateTime.Parse method for both the side for Property as well as Constant.

I tried following way but I am getting an error.

MethodInfo methodDateTimeParse = typeof(DateTime).GetMethod("Parse", new Type[] { typeof(string) });

Expression ExprLeft = Expression.Call(Expression.Property(variable, oFC.Field), methodDateTimeParse);  //Getiing error at this line (

Static method requires null instance, non-static method requires non-null instance.

Parameter name: instance

)

Please provide help

Thanks & Regards,

Somnath 

It looks like Expression.Property(...) is returning null. Are `variable` and `oFC.Field` correct?

Expression.Call(Expression.Property(...) ...) is the problem - that overload expects an instance of a class that you can call DateTime.Parse on - so it expects a DateTime object ... but Parse is static.  So you are supposed to give it a null instance - that means use the Expression.Call overload that does not take an Expression representing the instance.  Try this (partially tested):

MethodInfo methodDateTimeParse = typeof(DateTime).GetMethod("Parse", new Type[] { typeof(string) });
var variable = Expression.Parameter(typeof(string), "foo");    
Expression ExprLeft = Expression.Call(methodDateTimeParse, variable);

Here's a fully tested version:

MethodInfo methodDateTimeParse = typeof(DateTime).GetMethod("Parse", new Type[] { typeof(string) });
var variable = Expression.Parameter(typeof(string), "foo");
Expression exprLeft = Expression.Call(methodDateTimeParse, variable);
Expression<Func<string, DateTime>> lambda = Expression.Lambda<Func<string, DateTime>> (exprLeft, variable);
Func<string, DateTime> func = lambda.Compile();    
DateTime dt = func("2012-04-25");
Console.WriteLine(dt);

Hi Richard,

Thank you so much for your help.

The code which you gave that works fine with Constant which comes at the right side of the operator.

I have List<Report> lstReport=New List<Report>();

I want to write lstReport=lstReport.Where(o=>DateTime.Parse(o.Field)==DateTime.Parse(o.FieldValue));

I am creating above statement dynamically

like this 

  var variable = Expression.Variable(typeof(Report));

foreach (SQWFilterConstraint oFC in oFilter.LstFilterConstraint) //usingthis collection I am making dynamic query
{

  Expression  ExprLeft =Expression.Property(variable, oFC.Field);

MethodInfo methodDateTimeParse = typeof(DateTime).GetMethod("Parse", newType[] { typeof(string) });

var methodParam = Expression.Parameter(typeof(string), oFC.FieldValue);

Expression exprRight = Expression.Call(methodDateTimeParse, methodParam );

}

var props = new[] { variable };

var lambda = Expression.Lambda<Func<Report, bool>>(ExprPrev, props).Compile();

ReportList = ReportList.Where(lambda).ToList();

So Need to apply DateTime.Parse method on filed also which comes at the left side of the operator

I will be very thank full if you provide help.

Thanks in Advance 

@somnath2107:

This is the best I can do for you right now:

MethodInfo methodDateTimeParse = typeof(DateTime).GetMethod("Parse", new Type[] { typeof(string) });
var variable = Expression.Parameter(typeof(string), "foo");
Expression exprLeft = Expression.Call(methodDateTimeParse, variable);
ConstantExpression exprRight = Expression.Constant(DateTime.Parse("2012-01-01"));
BinaryExpression comparison = Expression.Equal(exprLeft, exprRight);
Expression<Func<string, bool>> lambda = Expression.Lambda<Func<string, bool>> (comparison, variable);
ConstantExpression constant = Expression.Constant (DateTime.Parse("2012-01-01"));
Func<string, bool> func = lambda.Compile();    
bool datesUnEqual = func("2012-04-25");
Console.WriteLine(datesUnEqual);
bool datesEqual = func("2012-01-01");
Console.WriteLine(datesEqual);

var variable = Expression.Parameter(typeof(string), "foo");

I cant use like this because value "foo" should be taken from collection  at the runtime

 Expression  ExprLeft =Expression.Property(variable, oFC.Field);

Can we apply DateTime.Parse to above expression ?

6 hours ago, somnath2107 wrote

var variable = Expression.Parameter(typeof(string), "foo");

I cant use like this because value "foo" should be taken from collection  at the runtime

 Expression  ExprLeft =Expression.Property(variable, oFC.Field);

Can we apply DateTime.Parse to above expression ?

Sorry I am swamped in the middle of a death march.  I suggest you ask the rest of your questions on Stackoverflow.com,  as you're bound to get more/better answers.

Good luck,

Richard

Am I the only one here that thinks this method of solving this problem is overkill? If you want to dynamically restrict a list of things, why not pass the list to some restriction function, based on whatever heuristics are available to you at runtime?

-Josh

Am I the only one here that thinks this method of solving this problem is overkill? If you want to dynamically restrict a list of things, why not pass the list to some restriction function, based on whatever heuristics are available to you at runtime?

-Josh

6 hours ago, JoshRoss wrote

Am I the only one here that thinks this method of solving this problem is overkill? If you want to dynamically restrict a list of things, why not pass the list to some restriction function, based on whatever heuristics are available to you at runtime?

-Josh

Overkill is the only way some people can think to solve a problem, especially if the code is non-readable to everybody except to themselves of course. It usually is an ego thing to make them think they are smart. I'm not saying this is the case here, but it usually is.

How is it overkill? It's not a lot of code. And it will be similar nearly everytime. Expression trees are the way to create dynamic LINQ ... it's core to LINQ ... so how can you do it any easier, such that it can be serialized into an expression tree across whatever provider you are targeting?

@Richard.Hein: I could understand using dynamic LINQ, if you wanted to create a LINQ provider. But in most cases, people just want to pass around IEnumerables. Sometimes it's hard to tell the forest from the trees.

-Josh