Creating dynamic queries using ParameterExpression in Odata Client

Question

I'm consuming an Odata v4 service using Odata Client Code generator proxy class.

MetroContext = new MetroContainer(new Uri(@"http://localhost:56222/service"));
IQueryable<Patient> query = MetroContext.Patients;
query = query.Where(x => x.FirstName == "john");

Above code is working fine. But I need to build queries dynamically. So I have tried following:

MetroContext = new MetroContainer(new Uri(@"http://localhost:56222/service"));
IQueryable<Patient> query = MetroContext.Patients;

ParameterExpression pe = Expression.Parameter(typeof(Patient), "patient");
Expression left = Expression.Property(pe, "FirstName");
Expression right = Expression.Constant("john");
Expression predicateBody = Expression.Equal(left, right);

query = query.Provider.CreateQuery<Patient>(predicateBody);

When I run the program I'm getting an error message:

Error translating Linq expression to URI: The binary operator 'Equal' is not supported.

Why I'm getting this error and how can I solve this?
How can I create dynamic queries by combining methods such as Contains() , StartsWith() ?

Ivan Stoev · Accepted Answer · 2016-03-29 19:24:16Z

2

I can't test that specific query provider, but at the first place predicateBody is not a valid expression for any provider.

Instead, you need to build Expression<Func<Patient, bool>> expression and pass it to the Where method like this:

// ...
var predicate = Expression.Lambda<Func<Patient, bool>>(predicateBody, pe);
query = query.Where(predicate);

What about your second question, you need to use some of the Expression.Call overloads. For instance:

Expression predicateBody = Expression.Call(left, "Contains", null, right);

Similar for "StartsWith" and "EndWith" etc.

answered Mar 29, 2016 at 19:24

Ivan Stoev

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7 Comments

Rahul Over a year ago

thanks, it's working perfectly. One more question, Can I use Equal, GreaterThan comparison in Expression.Call() method?

Ivan Stoev Over a year ago

No. Call is for methods. But of course you can use Equal, GreaterThan etc. in method arguments (for conditions). Do you have a specific example?

Rahul Over a year ago

Can I replace the code Expression.NotEqual(left, right) with something like Expression.Call(left, "NotEqual", null, right) ?

Ivan Stoev Over a year ago

No. These are operators, not methods.

Ivan Stoev Over a year ago

@Matt Not sure exactly about the question. If you have problems with the "normal" PredicateBuilder, you might check my answer to Establish a link between two lists in linq to entities where clause or the Universal PredicateBuilder

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