LINQ to Entities区分大小写比较

Question

这不是LINQ to Entities中区分大小写的比较:

Thingies.First(t => t.Name == "ThingamaBob");

如何与LINQ to Entities实现区分大小写的比较？

Answer 1

那是因为您正在使用LINQ To Entities,它最终将您的Lambda表达式转换为SQL语句.这意味着区分大小写受SQL Server的支配,默认情况下,SQL Server具有SQL_Latin1_General_CP1_CI_AS Collat​​ion,并且不区分大小写.

使用ObjectQuery.ToTraceString查看实际提交给SQL Server的生成的SQL查询揭示了一个谜团:

string sqlQuery = ((ObjectQuery)context.Thingies
        .Where(t => t.Name == "ThingamaBob")).ToTraceString();

当您创建LINQ to Entities查询时,LINQ to Entities利用LINQ解析器开始处理查询并将其转换为LINQ表达式树.然后将LINQ表达式树传递给Object Services API,后者将表达式树转换为命令树.然后将其发送到商店提供商(例如SqlClient),后者将命令树转换为本机数据库命令文本.查询得到的数据存储执行,结果被物化为实体对象通过对象服务.在考虑区分大小写之间没有任何逻辑.因此,无论您在谓词中添加什么案例,除非您更改该列的SQL Server Collat​​es,否则它将始终视为您的SQL Server.

服务器端解决方案

因此,最好的解决方案是将Thingies表中Name列的排序规则更改为COLLATE Latin1_General_CS_AS,它通过在SQL Server上运行它来区分大小写:

ALTER TABLE Thingies
ALTER COLUMN Name VARCHAR(25)
COLLATE Latin1_General_CS_AS

有关SQL Server Collat​​es的更多信息,请查看SQL SERVER Collat​​e区分大小写的SQL查询搜索

客户端解决方案:

您可以在客户端应用的唯一解决方案是使用LINQ to Objects进行另一个看似不太优雅的比较:

Thingies.Where(t => t.Name == "ThingamaBob")
        .AsEnumerable()
        .First(t => t.Name == "ThingamaBob");

Answer 2

WHERE默认情况下,SQL Server中的条件不区分大小写.通过将列的默认排序规则(SQL_Latin1_General_CP1_CI_AS)更改为,使其区分大小写SQL_Latin1_General_CP1_CS_AS.

执行此操作的脆弱方法是使用代码.添加新的迁移文件,然后在Up方法中添加:

public override void Up()
{
   Sql("ALTER TABLE Thingies ALTER COLUMN Name VARCHAR(MAX) COLLATE SQL_Latin1_General_CP1_CS_AS NOT NULL");
}

但

您可以使用新的EF6功能创建名为"CaseSensitive"的自定义注释,您可以像这样装饰您的属性:

[CaseSensitive]
public string Name { get; set; }

这篇博客文章解释了如何做到这一点.

Answer 3

您可以为EF6 + Code-first添加[CaseSensitive]注释

添加这个类

[AttributeUsage(AttributeTargets.Property, AllowMultiple = true)]
public class CaseSensitiveAttribute : Attribute
{
    public CaseSensitiveAttribute()
    {
        IsEnabled = true;
    }
    public bool IsEnabled { get; set; }
}

public class CustomSqlServerMigrationSqlGenerator : SqlServerMigrationSqlGenerator
{
    protected override void Generate(AlterColumnOperation alterColumnOperation)
    {
        base.Generate(alterColumnOperation);
        AnnotationValues values;
        if (alterColumnOperation.Column.Annotations.TryGetValue("CaseSensitive", out values))
        {
            if (values.NewValue != null && values.NewValue.ToString() == "True")
            {
                using (var writer = Writer())
                {
                    //if (System.Diagnostics.Debugger.IsAttached == false) System.Diagnostics.Debugger.Launch();

                    // https://github.com/mono/entityframework/blob/master/src/EntityFramework.SqlServer/SqlServerMigrationSqlGenerator.cs
                    var columnSQL = BuildColumnType(alterColumnOperation.Column); //[nvarchar](100)
                    writer.WriteLine(
                        "ALTER TABLE {0} ALTER COLUMN {1} {2} COLLATE SQL_Latin1_General_CP1_CS_AS {3}",
                        alterColumnOperation.Table,
                        alterColumnOperation.Column.Name,
                        columnSQL,
                        alterColumnOperation.Column.IsNullable.HasValue == false || alterColumnOperation.Column.IsNullable.Value == true ? " NULL" : "NOT NULL" //todo not tested for DefaultValue
                        );
                    Statement(writer);
                }
            }
        }
    }
}

public class CustomApplicationDbConfiguration : DbConfiguration
{
    public CustomApplicationDbConfiguration()
    {
        SetMigrationSqlGenerator(
            SqlProviderServices.ProviderInvariantName,
            () => new CustomSqlServerMigrationSqlGenerator());
    }
}

修改你的DbContext,添加

protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Conventions.Add(new AttributeToColumnAnnotationConvention<CaseSensitiveAttribute, bool>(
                "CaseSensitive",
                (property, attributes) => attributes.Single().IsEnabled));
        base.OnModelCreating(modelBuilder);
    }

然后做

添加迁移CaseSensitive

更新数据库

基于文章https://milinaudara.wordpress.com/2015/02/04/case-sensitive-search-using-entity-framework-with-custom-annotation/并修复了一些错误

Answer 4

@Morteza Manavi 给出的答案解决了这个问题。尽管如此，对于客户端解决方案，以下是一种优雅的方法（添加双重检查）。

var firstCheck = Thingies.Where(t => t.Name == "ThingamaBob")
    .FirstOrDefault();
var doubleCheck = (firstCheck?.Name == model.Name) ? Thingies : null;