Grid Filtering after State Restore Throws NullReferenceException

Environment

Product

Grid for Blazor, TreeList for Blazor

Description

I see errors when I load the Grid state at initialization (OnStateInit event handler) and then try to filter.

Steps to Reproduce

1. Load (restore) the Grid state from a serialized Json string in the OnStateInit handler.
2. The serialized state should contain filter descriptors.
3. Open a filter menu.

Error Message

The exception messages can vary, for example:

System.NullReferenceException: Object reference not set to an instance of an object.
    at Telerik.Blazor.Common.Filter.FilterOperatorFactory.GetFilterOperatorsForType(Type type, ITelerikStringLocalizer localizer)

Or alternatively:

System.NullReferenceException: Object reference not set to an instance of an object.
    at Telerik.Blazor.Components.Common.Filters.FilterList.TelerikFilterList.GetFilterOperators()
    at Telerik.Blazor.Components.Common.Filters.FilterList.TelerikFilterList.InitFilterOperators()

If the Grid is bound to OData, the OData query may be incorrect and the following exception may occur:

A binary operator with incompatible types was detected. Found operand types '...' and '...' for operator kind '...'.", "type": "Microsoft.OData.ODataException".

Possible Cause

The FilterDescriptor class has a MemberType property that is of type Type. The default JsonSerializer is unable to serialize types. After state restore in OnStateInit, the Grid filter descriptors end up with null MemberType values. This omission causes the errors.

There is a public issue about filtering error after Grid state restore in OnStateInit. Follow the item to receive status updates.

Solution

All suggested options are demonstrated in the examples below.

• Restore the Grid State later than OnStateInit. For example, use OnAfterRenderAsync and the Grid SetStateAsync() method instead.
• Manually set the missing MemberType property values in the restored filter descriptors in OnStateInit. There is no need to include filter descriptors for all columns in the state object.
• Restore the Grid State in a try {} catch() {} block. The Grid OnStateInit event fires two times - once in the prerender phase and once in the render phase. The deserialization problem will occur only if the Grid state is restored in the prerender phase. It is possible to skip state restoration during prerender with a JSInterop call. This is not allowed during prerender, so it will trigger an InvalidOperationException and OnStateInit execution will abort. This approach is used in example Save and Load Grid State from Browser LocalStorage

There are serializers which support Type serialization, for example Newtonsoft Json.NET. They, however, can cause other undesired side effects.

Examples

Restore the Grid state in OnAfterRenderAsync

@using System.Text.Json

<TelerikGrid @ref="@GridRef" />

@code {
    private TelerikGrid<GridModel> GridRef { get; set; }

    private string SerializedGridState { get; set; }

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            var gridState = JsonSerializer.Deserialize<GridState<GridModel>>(SerializedGridState);
            await GridRef.SetStateAsync(gridState);
        }

        await base.OnAfterRenderAsync(firstRender);
    }
}

Restore the Grid state and set MemberType manually

@using System.Text.Json

<TelerikGrid OnStateInit="@( (GridStateEventArgs<GridModel> args) => OnGridStateInit(args) )" />

@code {
    private string SerializedGridState { get; set; }

    private async Task OnStateInitHandler(GridStateEventArgs<GridModel> args)
    {
        var gridState = JsonSerializer.Deserialize<GridState<GridModel>>(SerializedGridState);

        var itemType = typeof(GridModel);

        foreach (CompositeFilterDescriptor cfd in gridState.FilterDescriptors)
        {
            foreach (FilterDescriptor fd in cfd.FilterDescriptors)
            {
                fd.MemberType = itemType.GetProperty(fd.Member).PropertyType;
            }
        }

        args.GridState = gridState;
    }
}

Restore the Grid state in the Blazor render phase

@using System.Text.Json
@inject IJSRuntime js

<TelerikGrid OnStateInit="@( (GridStateEventArgs<GridModel> args) => OnGridStateInit(args) )" />

<!-- suppress-error allows script tags in a .razor file. Avoid in production apps. -->
<script suppress-error="BL9992">function foo() {}</script>

@code {
    private string SerializedGridState { get; set; }

    private async Task OnGridStateInit(GridStateEventArgs<GridModel> args)
    {
        try
        {
            await js.InvokeVoidAsync("foo");
            args.GridState = JsonSerializer.Deserialize<GridState<GridModel>>(SerializedGridState);
        }
        catch (InvalidOperationException e)
        {

        }
    }
}

See Also

• Grid State documentation
• Filter Descriptors