Getting No generic method 'where' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments

Question

Getting No generic method 'where' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments

.net c# expression-trees json

Question

I have been writing expression tree for creating lambda expressions dynamically.

To start it up small, I am reading data from a JSON file into an Class object. And trying to build the where condition on two conditions.

string jsonnew = File.ReadAllText(@"C:\Users\nested_Json.txt");

var rootObject1 = JsonConvert.DeserializeObject<Citizen>(jsonnew);

IEnumerable<Citizen> enumerable = new[] { rootObject1 };

IQueryable<Citizen> queryableDataaa = enumerable.AsQueryable();

        ParameterExpression pe1 = Expression.Parameter(typeof(string), "rootobject");

        PropertyInfo field = typeof(Citizen).GetProperty("name");
        PropertyInfo field2 = typeof(Citizen).GetProperty("id");
        ParameterExpression targetExp = Expression.Parameter(typeof(Citizen), "rootobject");
        ParameterExpression valueExp = Expression.Parameter(typeof(string), "\"StrozeR\"");
        ParameterExpression valueExp2 = Expression.Parameter(typeof(Int32),"1765116");
        MemberExpression fieldExp = Expression.Property(targetExp, field);
        MemberExpression fieldExp2 = Expression.Property(targetExp, field2);
       Expression assignExp = Expression.Equal(fieldExp, valueExp);
       Expression assignExp2 = Expression.Equal(fieldExp2, valueExp2);

Expression predicateBody1 = Expression.AndAlso(assignExp, assignExp2);

MethodCallExpression whereCallExpression1 = Expression.Call(
            typeof(Queryable),
            "where",
            new Type[] { queryableDataaa.ElementType},
            queryableDataaa.Expression,
            Expression.Lambda<Func<string, bool>>(predicateBody1, new ParameterExpression[] { pe1 }));

Can anyone help me out in understand why I am getting

No generic method 'where' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic. error

Arjun

Popular Answer

You must

use constant expressions for your values
use one argument with type Citizen for the whole where clause expression
find the Where(with capital w) method of IQueryable where is an extension method defined in System.Linq.Queryable, then provide it's generic type
invoke the method with appropriate parameters

here is the corrected code:

var targetExp = Expression.Parameter(typeof(Citizen), "rootobject");
var valueExp = Expression.Constant("\"StrozeR\"");
var valueExp2 = Expression.Constant(1765116);
var fieldExp = Expression.Property(targetExp, "name");
var fieldExp2 = Expression.Property(targetExp, "id");
var assignExp = Expression.Equal(fieldExp, valueExp);
var assignExp2 = Expression.Equal(fieldExp2, valueExp2);
var predicateBody1 = Expression.AndAlso(assignExp, assignExp2);

var queryableType = typeof(System.Linq.Queryable);
var whereMethod = queryableType.GetMethods()
       .First(m =>
       {
           var parameters = m.GetParameters().ToList();
           //Put more restriction here to ensure selecting the right overload                
           //the first overload that has 2 parameters
           return m.Name == "Where" && m.IsGenericMethodDefinition &&
                 parameters.Count == 2;
       });
var whereClause = Expression.Lambda<Func<Citizen, bool>>(predicateBody1, 
                  new ParameterExpression[] { targetExp });
var genericMethod = whereMethod.MakeGenericMethod(typeof(Citizen));
var newQuery = (IQueryable<Citizen>)genericMethod
              .Invoke(genericMethod, new object[] { queryableDataaa, whereClause });

Taher Rahgooy

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