public class GridDataRowCommandContext :
    GridDataRowCommandContextBase,
    IGridCommandContext,
    ICommandContextBase

@inject WeatherForecastService ForecastService

<DxGrid Data="@Data" ContextMenus="GridContextMenus.DataRow" CustomizeContextMenu="OnCustomizeContextMenu">
    <Columns>
        <DxGridSelectionColumn />
        <DxGridDataColumn FieldName="Date" DisplayFormat="D" />
        <DxGridDataColumn FieldName="TemperatureC" Caption="@("Temp. (\x2103)")" Width="120px" />
        <DxGridDataColumn FieldName="TemperatureF" Caption="@("Temp. (\x2109)")" Width="120px" />
        <DxGridDataColumn FieldName="Forecast" />
        <DxGridDataColumn FieldName="CloudCover" />
    </Columns>
</DxGrid>

@code {
    object Data { get; set; }

    protected override void OnInitialized() {
        Data = ForecastService.GetForecast();
    }

    void OnCustomizeContextMenu(GridCustomizeContextMenuEventArgs args) {
        if (args.Context is GridDataRowCommandContext rowContext
                            && rowContext.Column is IGridSelectionColumn) {
            args.Items.AddCustomItem("Select All", async () => {
                rowContext.Grid.BeginUpdate();
                await rowContext.Grid.SelectAllAsync();
                rowContext.Grid.EndUpdate();
            });
            args.Items.AddCustomItem("Deselect All", async () => {
                rowContext.Grid.BeginUpdate();
                await rowContext.Grid.DeselectAllAsync();
                rowContext.Grid.EndUpdate();
            });
            args.Items.AddCustomItem("Select All on Page", () => {
                rowContext.Grid.BeginUpdate();
                rowContext.Grid.SelectAllOnPage();
                rowContext.Grid.EndUpdate();
            });
            args.Items.AddCustomItem("Deselect All on Page", () => {
                rowContext.Grid.BeginUpdate();
                rowContext.Grid.DeselectAllOnPage();
                rowContext.Grid.EndUpdate();
            });
        }
    }
}

using System;

public class WeatherForecast {
    public DateTime Date { get; set; }
    public int TemperatureC { get; set; }
    public double TemperatureF => Math.Round((TemperatureC * 1.8 + 32), 2);
    public string Forecast { get; set; }
    public string CloudCover { get; set; }
    public bool Precipitation { get; set; }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

public class WeatherForecastService {
    private List<WeatherForecast> Forecast { get; set; }

    private static string[] CloudCover = new[] {
        "Sunny", "Partly cloudy", "Cloudy", "Storm"
    };

    Tuple<int, string>[] ConditionsForForecast = new Tuple<int, string>[] {
        Tuple.Create( 22 , "Hot"),
        Tuple.Create( 13 , "Warm"),
        Tuple.Create( 0 , "Cold"),
        Tuple.Create( -10 , "Freezing")
    };

    public WeatherForecastService() {
        Forecast = CreateForecast();
    }

    private List<WeatherForecast> CreateForecast() {
        var rng = new Random();
        DateTime startDate = DateTime.Now;
        return Enumerable.Range(1, 15).Select(index => {
            var temperatureC = rng.Next(-10, 30);
            return new WeatherForecast {
                Date = startDate.AddDays(index),               
                TemperatureC = temperatureC,
                CloudCover = CloudCover[rng.Next(0, 4)],
                Precipitation = Convert.ToBoolean(rng.Next(0, 2)),
                Forecast = ConditionsForForecast.First(c => c.Item1 <= temperatureC).Item2
            };
        }).ToList();
    }

    public IEnumerable<WeatherForecast> GetForecast() {
        return Forecast.ToArray();
    }
    // ...
}

// ...
builder.Services.AddSingleton<WeatherForecastService>();