[Parameter]
public RenderFragment<GridDataColumnCellEditTemplateContext> DataColumnCellEditTemplate { get; set; }

@using Microsoft.EntityFrameworkCore
@inject IDbContextFactory<NorthwindContext> NorthwindContextFactory

<DxGrid @ref="Grid"
        Data="DataSource"
        KeyFieldName="EmployeeId"
        EditMode="GridEditMode.EditRow">
    <Columns>
        <DxGridCommandColumn />
        <DxGridDataColumn FieldName="FirstName" />
        <DxGridDataColumn FieldName="LastName" />
        <DxGridDataColumn FieldName="Title" />
        <DxGridDataColumn FieldName="HireDate" />
    </Columns>
    <DataColumnCellEditTemplate>
        @{
            var employee = (Employee)context.EditModel;
        }
        <MyCustomValidationMessage EditTemplateContext="context">
            @switch(context.DataColumn.FieldName) {
                case "FirstName":
                    <DxTextBox @bind-Text="@employee.FirstName" CssClass="w-100" ShowValidationIcon="false" />
                    break;
                case "LastName":
                    <DxTextBox @bind-Text="@employee.LastName" CssClass="w-100" ShowValidationIcon="false" />
                    break;
                case "Title":
                    <DxTextBox @bind-Text="@employee.Title" CssClass="w-100" ShowValidationIcon="false" />
                    break;
                case "HireDate":
                    <DxDateEdit @bind-Date="@employee.HireDate" CssClass="w-100" ShowValidationIcon="false" />
                    break;
                default:
                    throw new NotImplementedException();
            }
        </MyCustomValidationMessage>
    </DataColumnCellEditTemplate>
</DxGrid>

@code {
    IEnumerable<object> Data { get; set; }
    NorthwindContext Northwind { get; set; }
    DateTime DateTimeValue { get; set; } = DateTime.Today;
    bool CalendarVisible { get; set; }

    void OnChangeDayButtonClick(bool isAdd) {
        CalendarVisible = false;
        DateTimeValue = DateTimeValue.AddDays(isAdd ? 1 : -1);
    }
    // ...
}

@{
    var message = GetMessage();
}
<div class="d-flex align-items-center">
    @if(!string.IsNullOrWhiteSpace(message)) {
            <div class="grid-validation-message bg-danger" title="@message">
            @ChildContent
            </div>
    }
    else
        @ChildContent
</div>

@code {
    [Parameter]
    public GridDataColumnCellEditTemplateContext EditTemplateContext { get; set; }
    [Parameter]
    public RenderFragment ChildContent { get; set; }
    string GetMessage() {
        var field = EditTemplateContext.EditContext.Field(EditTemplateContext.DataColumn.FieldName);
        return string.Join("\n", EditTemplateContext.EditContext.GetValidationMessages(field));
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;

namespace Grid.Northwind {
    public partial class Employee {
        public int EmployeeId { get; set; }
        [Required]
        public string LastName { get; set; }
        [Required]
        public string FirstName { get; set; }
        [Required]
        public string Title { get; set; }
        public string TitleOfCourtesy { get; set; }
        public Nullable BirthDate { get; set; }
        [Required]
        [Range(typeof(DateTime), "1/1/2000", "1/1/2020",
        ErrorMessage = "HireDate must be between {1:d} and {2:d}")]
        public Nullable HireDate { get; set; }
        public string Address { get; set; }
        public string City { get; set; }
        public string Region { get; set; }
        public string PostalCode { get; set; }
        public string Country { get; set; }
        public string HomePhone { get; set; }
        public string Extension { get; set; }
        public byte[] Photo { get; set; }
        public string Notes { get; set; }
        public Nullable<int> ReportsTo { get; set; }
        public string PhotoPath { get; set; }
        public virtual ICollection<Order> Orders { get; set; }
        public string Text => $"{FirstName} {LastName} ({Title})";
        public string ImageFileName => $"Employees/{EmployeeId}.jpg";
    }
}

using Microsoft.EntityFrameworkCore;
#nullable disable

namespace Grid.Northwind {
    public partial class NorthwindContext : DbContext {

        public NorthwindContext(DbContextOptions<NorthwindContext> options)
            : base(options) {
        }
        // ...
        public virtual DbSet<Employee> Employees { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) {
            if(!optionsBuilder.IsConfigured) {
                optionsBuilder.UseSqlServer("Server=.\\sqlexpress;Database=Northwind;Integrated Security=true");
            }
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder) {
            modelBuilder.HasAnnotation("Relational:Collation", "SQL_Latin1_General_CP1_CI_AS");
            // ...
            modelBuilder.Entity<Employee>(entity => {
                entity.HasIndex(e => e.EmployeeId, "EmployeeId");
                entity.HasIndex(e => e.LastName, "LastName");
                entity.HasIndex(e => e.FirstName, "FirstName");
                entity.HasIndex(e => e.Title, "Title");
                entity.HasIndex(e => e.BirthDate, "BirthDate");
                entity.HasIndex(e => e.HireDate, "HireDate");
                entity.HasIndex(e => e.Notes, "Notes");
            });
            OnModelCreatingPartial(modelBuilder);
        }

        partial void OnModelCreatingPartial(ModelBuilder modelBuilder);
    }
}

using Microsoft.EntityFrameworkCore;
// ...
builder.Services.AddDbContextFactory<NorthwindContext>((sp, options) => {
    var env = sp.GetRequiredService<IWebHostEnvironment>();
    var dbPath = Path.Combine(env.ContentRootPath, "Northwind.mdf");
    options.UseSqlServer("Server=(localdb)\\MSSQLLocalDB;Integrated Security=true;AttachDbFileName=" + dbPath);
});