ASP.NET Core : how to sum the column based on another tables's column

Question

I am new to ASP.NET Core.

public List<LineItem> LineItems { get; set; }

public double Total => this.LineItems.Sum(x => x.LineTotal);

LineItem

public int Quantity { get; set; }
public double UnitPrice { get; set; }
public double LineTotal => Math.Round(this.Quantity * this.UnitPrice, 2);

The error said Sum() is not used properly.

ArgumentNullException: Value cannot be null. (Parameter 'source')

Answer 1

I think this error may be due to Object reference not set to an instance of an object,you could try to modify the code:

public List<LineItem> LineItems { get; set; }

to

public List<LineItem> LineItems =new List<LineItem>???

And I get the excepted result after I changed the codes,If you have any other problems please tell me more details.

Result