MudSelect is not showing selected value - asp.net-core

Question

I am facing issue with MudSelect with MultiSelect="true". Even though it has the selected values set it does not check the selected value:
<MudSelect Label="Select Item" SelectAll="true" MultiSelection="true" SelectedValues="#SelectedTestItems" T="#MultiSelectItem" ToStringFunc="#ToString">
#foreach (MultiSelectItem item in AllItems)
{
<MudSelectItem Value="#item" />
}
</MudSelect>
#code {
private List<MultiSelectItem> SelectedTestItems = new() {
new MultiSelectItem{Id = 1, Name = "Test 1"},
};
private List<MultiSelectItem> AllItems = new() {
new MultiSelectItem{Id = 1, Name = "Test 1"},
new MultiSelectItem{Id = 2, Name = "Test 2"},
new MultiSelectItem{Id = 3, Name = "Test 3"},
new MultiSelectItem{Id = 4, Name = "Test 4"},
};
private string ToString(MultiSelectItem x)
=> x is null ? string.Empty : $"{x.Name}";
public class MultiSelectItem
{
public int Id { get; set; }
public string Name { get; set; }
}
}
I posted details #github also: https://github.com/MudBlazor/MudBlazor/discussions/3532.

Answer 1

Your problem is that you did not override necessary methods for equality comparison. Here is a solution based on your snippet, you can play with it online: https://try.mudblazor.com/snippet/wYwPbGvKRlGThZsa
MudSelect uses a HashSet internally, so when you use custom reference types as values you must make sure they have GetHashCode and Equals based on their data:
<MudSelect Label="Select Item" SelectAll="true" MultiSelection="true" SelectedValues="#SelectedTestItems" T="#MultiSelectItem" ToStringFunc="#ToString">
#foreach (MultiSelectItem item in AllItems)
{
<MudSelectItem Value="#item" />
}
</MudSelect>
#code {
private List<MultiSelectItem> SelectedTestItems = new() {
new MultiSelectItem{Id = 1, Name = "Test 1"},
};
private List<MultiSelectItem> AllItems = new() {
new MultiSelectItem{Id = 1, Name = "Test 1"},
new MultiSelectItem{Id = 2, Name = "Test 2"},
new MultiSelectItem{Id = 3, Name = "Test 3"},
new MultiSelectItem{Id = 4, Name = "Test 4"},
};
private string ToString(MultiSelectItem x)
=> x is null ? string.Empty : $"{x.Name}";
public class MultiSelectItem
{
public int Id { get; set; }
public string Name { get; set; }
public override bool Equals(object o) {
var other = o as MultiSelectItem;
return other?.Id==Id;
}
public override int GetHashCode() => Id.GetHashCode();
}
}