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dropdowns-readonly-struct-error.md

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title description type page_title slug position tags ticketid res_type
ComboBox does not work when the data is readonly structs
Using readonly struct objects for the Data of the dropdowns causes errors
troubleshooting
Readonly Struct data source breaks the dropdowns
dropdowns-kb-readonly-struct-error
1489058
kb

Environment

Product ComboBox for Blazor, DropDownList for Blazor, Autocomplete for Blazor, MultiSelect for Blazor

Description

When using the combo box component when the desired data is list of readonly structs there is a null reference exception and the combo box does not work.

Error Message

A typical error would be null reference exception like the one below:

NullReferenceException: Object reference not set to an instance of an object.
Telerik.Blazor.Components.TelerikComboBox<TItem, TValue>.<OnParametersSetAsync>b__70_0(ListDataItem item)

Steps to Reproduce

````RAZOR Selected value: @selectedValue

<TelerikComboBox Data="@myComboData" TextField="MyTextField" ValueField="MyValueField" @bind-Value="selectedValue" Placeholder="Select an item..." ShowClearButton="true" Filterable="true">

@code { public readonly struct ModelData { public ModelData(string textField, int valueField) { MyTextField = textField; MyValueField = valueField; } public readonly string MyTextField; public readonly int MyValueField; }

int selectedValue { get; set; } = 3;

IEnumerable<ModelData> myComboData = Enumerable.Range(1, 20).Select(x => new ModelData("item " + x, x));

}


## Possible Cause
The components require a model when binding so it can be instantiated with a parameterless constructor. This is a requirement that comes down to the forms validation that they must support and getting the `Default` value and object. Structs do not have a parameterless constructor.

## Solution
There are two approaches to avoiding this error:

* Use a model (class), not a struct, as shown [here](slug:components/combobox/databind#bind-to-a-model), for example

* When setting the `Data` of the dropdown, make it a collection of anonymous objects, for example:

<div class="skip-repl"></div>
````RAZOR.skip-repl
@* See the Select in the Data parameter *@

<TelerikComboBox Data="@myComboData.Select(x => new {MyTextField = x.MyTextField, MyValueField = x.MyValueField })" 
                TextField="MyTextField" ValueField="MyValueField" 
                @bind-Value="selectedValue"
                Placeholder="Select an item..." ShowClearButton="true" Filterable="false">
</TelerikComboBox>