[Parameter]
public EventCallback<CalculateDocumentVariableEventArgs> CalculateDocumentVariable { get; set; }

<DxRichEdit @bind-Selection="@selection"
            @ref="@richEdit"
            CalculateDocumentVariable="@(async x => await OnCalculateDocumentVariable(x))" />

@code {
    DxRichEdit richEdit { get; set; }
    Selection selection { get; set; }
    protected override Task OnAfterRenderAsync(bool firstRender) {
        if(firstRender)
            InitializeDocument();
        return base.OnAfterRenderAsync(firstRender);
    }

    async Task InitializeDocument() {
        /* Surround the code that contains an asynchronous operation with a try-catch block to handle
        the OperationCanceledException. This exception is thrown when an asynchronous operation is canceled. */
        var documentAPI = richEdit.DocumentAPI;
        documentAPI.BeginUpdate();
        try {
            await documentAPI.Paragraphs.CreateAsync();
            var textSpan = await documentAPI.AddTextAsync("Number of paragraphs: ");
            var field = await documentAPI.Fields.CreateAsync(textSpan.Interval.Start + textSpan.Interval.Length, "DOCVARIABLE paragraphCount");
            await documentAPI.Fields.UpdateAsync(field);
        } catch(OperationCanceledException e) {
            Console.WriteLine($"{nameof(OperationCanceledException)} thrown with message: {e.Message}");
        }
        documentAPI.EndUpdate();
    }
    async Task OnCalculateDocumentVariable(CalculateDocumentVariableEventArgs eventArgs) {
        try {
            switch(eventArgs.VariableName) {
                case "paragraphCount":
                    var paragraphs = await richEdit.DocumentAPI.Paragraphs.GetAllAsync();
                    eventArgs.Value = paragraphs.Count.ToString();
                    break;
                default:
                    break;
            }
        } catch(OperationCanceledException e) {
            Console.WriteLine($"{nameof(OperationCanceledException)} thrown with message: {e.Message}");
        }
    }
}