Why expression is not working in linq

Question

Work on EF,Need to write a custom where

public virtual IEnumerable GetDataByID<TEntity>() where TEntity : class
{
 if (this.Context == null)
 Initialize<TEntity>();
 TContext context = this.Context;
 var result = context.CreateObjectSet<TEntity>().Where(GetExpression<TEntity>(5));
 return result;
}
public Expression GetExpression<TEntity>(int id) where TEntity : class
{
 ParameterExpression e = Expression.Parameter(typeof(TEntity), "e");
 PropertyInfo propInfo = typeof(TEntity).GetProperty(EntityInfo<TEntity>.GetPkFieldName(this.Context));//(KeyPropertyName);
 MemberExpression m = Expression.MakeMemberAccess(e, propInfo);
 ConstantExpression c = Expression.Constant(id, typeof(int));
 BinaryExpression b = Expression.Equal(m, c);
 Expression<Func<TEntity, bool>> lambda = Expression.Lambda<Func<TEntity, bool>>(b, e);
 return lambda;
}

Error:

The best overloaded method match for 'System.Data.Objects.ObjectQuery.Where(string, params System.Data.Objects.ObjectParameter[])' has some invalid arguments

Why my above syntax is not working, how to fix it?

• Mikey Mouse: What's the error message?

Answer 1

The problem is, the result of CreateObjectSet<TEntity> returns an ObjectSet<TEntity> which defines its own Where method which the compiler will attempt to use. The arguments you are providing are not valid for this method, hence the error.

What you want to be using is Linq's Queryable.Where extension method like so:

var predicate = GetExpression<TEntity>(5);
var objectSet = context.CreateObjectSet<TEntity>();
var results = System.Linq.Queryable.Where(objectSet, predicate);

Answer 2

The best overloaded method match for 'System.Data.Objects.ObjectQuery.Where(string, params System.Data.Objects.ObjectParameter[])' has some invalid arguments

The problem is this line:

var result = context.CreateObjectSet<TEntity>().Where(GetExpression<TEntity>(5));

Infact you're specifing the wrong parameters: ObjectQuery<T>.Where

public ObjectQuery<T> Where(
 string predicate,
 params ObjectParameter[] parameters
)

Infact you should pass a string.

• Lukazoid: I clicked to downvote before your edit, where you were comparing an entity to an expression. SO seems to have registered the downvote as occurring after the edit (minor glitch?) so I am currently unable to remove it. If you make a small edit I will remove the downvote, as I see nothing wrong with the answer now :)