Morshed Anwar's Blog

Generic method to query in Ado.net Entity Framework

Posted on March 24, 2009. Filed under: Entity Framework |

Here I am going to talk about the generic method and expression to query using Ado.net Entity Framework. I know it’s not a big deal but I choose simple thing to share my very little knowledge (!!) about expression and entity framework.Ok, here I consider an example of querying Max value. I know it’s already in there but it’s only an approach- of how you use your own (custom) expression when you are in generic place and  you don’t know much (properties) about your entity set:

public static int GetMaxValue  <  T  >  (ObjectContext contex, string keyFieldName)
{
   if (null != contex)
   {
     try{
       // First we define the parameter that we are going to use the clause.
        var param = Expression.Parameter(typeof(T), typeof(T).Name);
      // Now we’ll make our lambda function that returns the "ID" property .
       var maxExpression = Expression.Lambda< Func <  T ,int  > >(Expression.Property(param, keyFieldName), param);
       // Now I can get desire entityset from context using reflection.
      System.Data.Objects.ObjectQuery  <  T >    query = contex.GetType().GetProperties().Where(p => p.PropertyType.BaseType.Equals(typeof(System.Data.Objects.ObjectQuery)) && p.PropertyType.GetGenericArguments().First().Equals(typeof(T))).First().GetValue(contex, null) asSystem.Data.Objects.ObjectQuery <  T > ;
         if (null != query)
            {                     
                int? i = query.Max <  T ,  int >(maxExpression);
                if (i.HasValue)
                return i.Value;                  
             }
       }
       catch (Exception){}
      }
     return 0;
}

You can create your own expression in place of “maxExpression” . Now how do you call this generic method from generic place.lets consider that you want to get the Max of entity-key(Primary key) of an entity-set(Table) and set the (Max + 1) in your new entity object that is going to be added in database.Using reflection you can get the property name/names of Entity key

IEnumerable<PropertyInfo> keyProperties = entity.GetType().GetProperties().
    Where(p => ((EdmScalarPropertyAttribute[])p.GetCustomAttributes(typeof(EdmScalarPropertyAttribute), false)).Any(a => a.EntityKeyProperty == true));

If you have composite primary key then  value of “keyProperties” will be greater than one. For making our  example very simple I assume that here is no composite key here and the datatype of entity key is integer.

PropertyInfo item = keyProperties[0];
//type generic method 
Type typeofClass = typeof(ClassOfGenericMethod);

//take the specific static method 
MethodInfo methodInfo = typeofClass.GetMethod("GetMaxValue", BindingFlags.Public);

// Binding the method info to generic arguments 
Type[] genericArguments = new Type[] { entity.GetType() };
MethodInfo genericMethodInfo = methodInfo.MakeGenericMethod(genericArguments);
// Invoke the method and passing parameters 
// The null parameter is the object to call the method from.if it is static, pass null. 
object returnValue = genericMethodInfo.Invoke(null, new object[] { context, item.Name });
//set max value  
item.SetValue(entity, (long)result + 1, null); 

Here I call the “GetMaxValue” and set its value by incease of one to entity-key Property of target object. Anyways Sorry for my bad english.

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