Blazor Unhandled exception rendering component

Question

I'm a beginner at Blazor and am making a simple to-do list project. I got an error when adding a list. Unhandled exception rendering component: Object reference not set to an instance of an object.

System.NullReferenceException: Object reference not set to an instance of an object. at FrontEnd.Pages.Index.AddList(KeyboardEventArgs e) in C:\Users\bryan\source\repos\Productivity_App\FrontEnd\Pages\Index.razor.cs:line 20 at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

I tried searching for the problem and found a similar post but the answer didn't help me at all, looking at the question. I think it has something to do with blazor lifecycle to fix it. Here's the code related to the error.

Index.razor:

<div class="toDoList">
    @if (ToDoListCollection != null)
    {
        @foreach (string toDoList in ToDoListCollection)
        {
            <input type="checkbox" id="checkbox">
            <label for="checkbox">@toDoList</label>
            <hr />
        }
    }
    else {}
</div>

Index.razor.cs

public partial class Index
{
    public bool IsCompleted { get; set; }
    public string Description { get; set; }
    public List<string> ToDoListCollection { get; set; }
    
    public void AddList(KeyboardEventArgs e)
    {
        if (e.Key == "Enter" && !string.IsNullOrWhiteSpace(Description) || e.Key == "NumpadEnter" && !string.IsNullOrWhiteSpace(Description))
        {
            ToDoListCollection.Add($"{Description}"); // This is the line that the error message is referencing 
            Description = null;
        } 
        else {}
    }

}

Is the collection initialized? In the code you provided I don't see anything along the lines of ToDoListCollection = new List<string>() — Nijenhof
– Nijenhof, Commented Jan 27, 2021 at 5:30
Does this answer your question? What is a NullReferenceException, and how do I fix it? — Timothy G.
– Timothy G., Commented Apr 3, 2023 at 13:54

puko · Accepted Answer · 2021-01-27 05:29:19Z

7

I assume that your ToDoListCollection has null value because it is not inialized.

Assign a default value to your collection.

public List<string> ToDoListCollection { get; set; } = new List<string>();

answered Jan 27, 2021 at 5:29

puko

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