DropDownList Data Binding

This article explains the different ways to provide data to a DropDownList component, the properties related to data binding and their results.

For details on Value Binding and Data Binding, and the differences between them, see the Value Binding vs Data Binding article.

There are two key ways to bind data:

• Bind to Primitive Types
• Bind to a Model

and some considerations you may find useful, such as showing the DefaultText when the value is out of the data source range:

• Considerations
  • Value Out of Range
  • Component Reference
  • Missing Value or Data

Primitive Types

You can data bind the DropDownList to a simple collection of data. When you have a concrete list of options for the user to choose from, their string representation is often suitable for display and you do not need special models.

To bind the dropdownlist to a primitive type (like int, string, double), you need to

1. provide an IEnumerable<TItem> of the desired type to its Data property
2. set a corresponding Value. If the Value is null, it will be populated with the first item from the data source.

Data binding a DropDownList to a primitive type

Bind to a List of a primitive type (stirng, int,...)

<TelerikDropDownList Data="@MyList" @bind-Value="MyItem">
</TelerikDropDownList>

@code {
    protected List<string> MyList = new List<string>() { "first", "second", "third" };

    protected string MyItem { get; set; }

    //Define a preselected value when the component initializes
    protected override void OnInitialized()
    {
        MyItem = "second";
    }
}

Bind to a Model

You can bind the DropDownList to a model in your application. This is useful when you have a numerical representation of a finite list (for example, departments in a company), and you want the user to choose them based on a friendly text name.

To bind the DropDownList to a model:

1. Populate its Data parameter with the collection of items you want in the dropdown.
2. Set the TextField and ValueField parameters to point to the corresponding property names of the model.
3. Set the Value property to the intial value of the component (optional).

The TextField and ValueField parameters must point to model properties, which are of primitive type (int, string, etc.). The Value and ValueField types must match and also be primitive.

Data binding a DropDownList to a model

Bind to a collection of models

<TelerikDropDownList Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField" @bind-Value="selectedValue">
</TelerikDropDownList>

@code {
    //in a real case, the model is usually in a separate file
    //the model type and value field type must be provided to the dropdpownlist
    public class MyDdlModel
    {
        public int MyValueField { get; set; }
        public string MyTextField { get; set; }
    }

    int selectedValue { get; set; }

    //Define a preselected value when the component initializes
    protected override void OnInitialized()
    {
        selectedValue = 3;
    }

    IEnumerable<MyDdlModel> myDdlData = Enumerable.Range(1, 20).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x });
}

If you are looking for more fields from the view-model that describes the dropdown items, not just the Value, see the Get model from dropdown KB article and the OnChange event.

You may also want to review/join the discussion and Vote for this request: Binding DropDownList Value to complex model

Considerations

The DropDownList component attempts to infer the type of its model and value based on the provided Data and initial Value. This affects the way its reference is obtained and what happens if you can't provide data or a value. Providing a value that is not in the data source needs to be taken into account be the app, because the component will not change it.

Value Out of Range

When the Value the application provides does not match any of the values present in the ValueField of the Data collection, the DropDownList component will not change the Value or select a new item. In the common case, it will show up blank to indicate there is nothing selected from its data.

If you have set the DefaultText and the Value matches the default value of the type (for example, 0 for an int or null for an int? or string), you will see the DefaultText. A Value that is non-default will not show the DefaultText.

Handling such "unexpected" values is up to the application - for example, through defensive checks, or through form validation, or by first checking what is present in the data source before setting a new Value.

Component Reference

The DropDownList is a generic component and its type comes from the model it is bound to and from the value field type. When bound to a primitive type, the reference is of that primitive type only.

Reference type when binding to primitive values

<TelerikDropDownList @ref="myDdlRef" Data="@MyList" Value="@initialValue">
</TelerikDropDownList>

@code {
    //the type of the generic component is determined by the type of the model you pass to it, and the type of its value field
    Telerik.Blazor.Components.TelerikDropDownList<string, string> myDdlRef;

    protected List<string> MyList = new List<string>() { "first", "second", "third" };

    string initialValue { get; set; }

    //Define a preselected value when the component initializes
    protected override void OnInitialized()
    {
        initialValue = "third";
    }
}

Reference when binding to model collections

<TelerikDropDownList @ref="myDdlRef" Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField" Value="3">
</TelerikDropDownList>
@code {
    //the type of the generic component is determined by the type of the model you pass to it, and the type of its value field
    Telerik.Blazor.Components.TelerikDropDownList<MyDdlModel, int> myDdlRef;

    IEnumerable<MyDdlModel> myDdlData = Enumerable.Range(1, 20).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x });

    public class MyDdlModel
    {
        public int MyValueField { get; set; }
        public string MyTextField { get; set; }
    }
}

Missing Value or Data

In case you cannot provide either of a Value, or Data, or both when the component initializes, you need to set the corresponding type properties to the TItem and TValue properties as shown below.

DropDownList configuration if you cannot provide Value or Data

How to declare the dropdown if no Value or Data are provided

<TelerikDropDownList Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField" TValue="int" TItem="MyDdlModel">
</TelerikDropDownList>

@code {
    public class MyDdlModel //TItem matches the type of the model
    {
        public int MyValueField { get; set; } //TValue matches the type of the value field
        public string MyTextField { get; set; }
    }

    IEnumerable<MyDdlModel> myDdlData = Enumerable.Range(1, 20).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x });

    //the same configuration applies if the "myDdlData" object is null initially and is populated on some event
}

See Also

• DropDownList Overview
• Live Demo: DropDownList