MVC ViewModel - Object Reference Not Set to Instance of an Object

问题

I receive an "Object reference not set to instance of an object" error while trying to convert a DataTable into a viewmodel containing rows and columns.

ViewModel

public class ViewModel
{
        public List<ColumnViewModel> Columns { get; set; }
        public List<RowViewModel> Rows { get; set; }
}

    public class ColumnViewModel
    {
        public string Name { get; set; }
    }

    public class RowViewModel
    {
        public List<CellValueViewModel> Values { get; set; }
    }

    public class CellValueViewModel
    {
        public string Value { get; set; }
    }

Model

    ViewModel myViewModel = new ViewModel();
    CellValueViewModel myCellValueViewModel = new CellValueViewModel();
    RowViewModel myRowViewModel = new RowViewModel();
    foreach (DataColumn column in GridData.Columns)
    {
        ColumnViewModel myColumnViewModel = new ColumnViewModel();
        myColumnViewModel.Name = column.ColumnName;
        myViewModel.Columns.Add(myColumnViewModel);
    }

The error occurs at myViewModel.Columns.Add(myColumnViewModel); I do not understand why this is happening since Ive instantiated each object that Im using here.

回答1:

Well, you never assigned the Columns property a value. Make sure you have instantiated it before attempting to add values to this list:

ViewModel myViewModel = new ViewModel();
myViewModel.Columns = new List<ColumnViewModel>();

来源：https://stackoverflow.com/questions/15731440/mvc-viewmodel-object-reference-not-set-to-instance-of-an-object