[聚合问答] LINQ to Entities case sensitive comparison

c#,.net,entity-framework-4,linq-to-entities 2017-11-30 41 阅读

This isn't a case-sensitive comparison in LINQ to Entities:

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

How can I achieve case sensitive comparison with LINQ to Entities?

3个回答

135

That's because you are using LINQ To Entities which is ultimately convert your Lambda expressions into SQL statements. That means the case sensitivity is at the mercy of your SQL Server which by default has SQL_Latin1_General_CP1_CI_AS Collation and that is NOT case sensitive.

Using ObjectQuery.ToTraceString to see the generated SQL query that has been actually submitted to SQL Server reveals the mystery:

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

When you create a LINQ to Entities query, LINQ to Entities leverages the LINQ parser to begin processing the query and converts it into a LINQ expression tree. The LINQ expression tree is then passed to Object Services API, which converts the expression tree to a command tree. It is then sent to the store provider (e.g. SqlClient), which convert the command tree into the native database command text. Query get executed on the data store and the results are Materialized into Entity Objects by Object Services. No logic has been put in between to take case sensitivity into account. So no matter what case you put in your predicate, it will always treat as the same by your SQL Server unless you change your SQL Server Collates for that column.

Server side solution:

Therefore, the best solution would be to change the collation of the Name column in the Thingies table to COLLATE Latin1_General_CS_AS which is case sensitive by running this on your SQL Server:

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

For more information on the SQL Server Collates, take a a look at SQL SERVER Collate Case Sensitive SQL Query Search

Client-side solution:

The only solution that you can apply on client side is to use LINQ to Objects to do yet another comparison which doesn't seem to be very elegant:

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

2017-11-30
9

You can add [CaseSensitive] annotation for EF6+ Code-first

Add this classes

[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());
    }
}

Modify your DbContext, add

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

Then do

Add-Migration CaseSensitive

Update-Database

based on article https://milinaudara.wordpress.com/2015/02/04/case-sensitive-search-using-entity-framework-with-custom-annotation/ with some bug fix

2017-11-30
8

WHERE conditions in SQL Server are case insensitive by default. Make it case sensitive by changing the column's default collations (SQL_Latin1_General_CP1_CI_AS) to SQL_Latin1_General_CP1_CS_AS.

The fragile way to do this is with code. Add a new migration file and then add this inside the Up method:

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

But

You can create custom annotation called "CaseSensitive" using the new EF6 features and you can decorate your properties like this:

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

This blog post explains how to do that.

2017-11-30

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