ModelValidationException was unhandled user code

Can you help me on the following error, i checked everything no ID mistakes

ModelValidationException was un handled by user code

An exception of type ‘System.Data.Entity.ModelConfiguration.ModelValidationException’ occurred in EntityFramework.dll but was not handled in user code

Additional information: One or more validation errors were detected during model generation:

 public int GetCount()
    {
        ShoppingCartId = GetCartId();

        // Get the count of each item in the cart and sum them up          
        int? count = (from cartItems in _db.ShoppingCartItems
                        where cartItems.CartId == ShoppingCartId
                        select (int?)cartItems.Quantity).Sum();
        // Return 0 if all entries are null         
        return count ?? 0;
    }

Answers:

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Method 1

Frustratingly .net does not always show you the inner exception. Wrap your code in a try block with this catch

catch (DbEntityValidationException ex) {
    // Retrieve the error messages as a list of strings.
    var errorMessages = ex.EntityValidationErrors
            .SelectMany(x => x.ValidationErrors)
            .Select(x => x.ErrorMessage);

    // Join the list to a single string.
    var fullErrorMessage = string.Join("; ", errorMessages);

    // Combine the original exception message with the new one.
    var exceptionMessage = string.Concat(ex.Message, " The validation errors are: ", fullErrorMessage);

    // Throw a new DbEntityValidationException with the improved exception message.
    throw new DbEntityValidationException(exceptionMessage, ex.EntityValidationErrors); }

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