Cannot make a dynamic LINQ query work

sun21170 · September 2014

I am using the following dynamic query in LinqPad with PredicateBuilder enabled. You can enable this by pressing F4 and then checking the check-box in bottom right of the popup.

The query throws this error when I try to execute it: Cannot implicitly convert type 'string' to 'System.Linq.Expressions.Expression>'

Question: What is the problem with my dynamic LINQ query below? I had C#Expressions selected from LinqPad drop-down.

I am trying to dynamically build a where condition for a LINQ query. Query is based on standard Northwind database.

<code>int? orderParam = 100;
string orderOperator = ">=";
string linqFilter = "";
linqFilter= String.Format("{0} {1} {2}", "o.OrderID", orderOperator, orderParam);
linqFilter.Dump();
 var predicate = PredicateBuilder.False<Orders>();
 predicate = (linqFilter);
 var dynamicResult = from o in Orders.Where(predicate) select o;
 dynamicResult.Dump

Sorax · September 2014

PredicateBuilder can't do exactly what you want here. As the exception says, you need an Expression not a string. And the operator can't be dynamic. The following would be the closest:

int? orderParam = 100;
string orderOperator = ">=";
var predicate = PredicateBuilder.True<Orders>();

if(orderOperator == ">=")
    predicate = predicate.And(a => a.OrderID >= orderParam);

var result = from o in Orders.Where(predicate) select o;
result
.Dump();

Running in C# Statement(s) is required for this code.

Cannot make a dynamic LINQ query work

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