c# - 如何使用 Entity Framework `ValueComparer<T>` ？

Question

我正在处理一个 ASP.NET 3.1 项目。 类(class)EmployeeSummary包含公共(public)属性(property) string[] Roles

public string[] Roles { get; set; }

并为字符串数组设置了值转换器:

          modelBuilder.Entity<EmployeeSummary>()
                .Property(e => e.Roles)
                .HasConversion(v => string.Join(",", v),
                    v => v.Split(new[]
                        {
                            ","
                        }, StringSplitOptions.RemoveEmptyEntries)
                        .ToList()
                        .Distinct(StringComparer.OrdinalIgnoreCase)
                        .ToArray());

我得到以下 System.InvalidOperationException表示我没有设置值比较器:

The property 'Roles' on entity type 'EmployeeSummary' is a collection or enumeration type 
with a value converter but with no value comparer. 
Set a value comparer to ensure the collection/enumeration elements are compared correctly.

我的方法是添加(类似于 Microsoft documentation 中的示例)

• A var valueComparer在类里面EmployeeSummary
• 设置为 new ValueComparer<string[]>而不是 new ValueComparer<List<int>>
• 检查两个数组是否相同。 然而，这不起作用:

        public string[] Roles { get; set; }

        private var valueComparer = new ValueComparer<string[]>(
             (c1, c2) => c1.SequenceEqual(c2));

带有无法解析构造函数的消息。这是正确的方法吗，或者我该如何解决此错误消息？

Answer 1

您需要在方法HasConversion 中设置ValueComparer。

一个可行的例子是:

            modelBuilder.Entity<EmployeeSummary>()
            .Property(e => e.Roles)
            .HasConversion(
                v => string.Join(',', v),
                v => v.Split(',', StringSplitOptions.RemoveEmptyEntries).ToList()
                    .Distinct(StringComparer.OrdinalIgnoreCase)
                    .ToArray(), new ValueComparer<ICollection<string>>(
                    (c1, c2) => c1.SequenceEqual(c2),
                    c => c.Aggregate(0, (a, v) => HashCode.Combine(a, v.GetHashCode())),
                    c => c.ToList()));