Sequence contains no matching element - EntityFramework

问题

I'm using EF 6.1.0 and was creating a WCF Service.

First I created a Class Library containing my entities, Mappers and Context for initializing EF. I've also created a class containing an instantiation of the context and have this code:

public IQueryable<[Entity]> GetAll()
{
    return context.[Entity].AsQueryable();
}

On the otherhand, I've created a WCF Service on the same project and Calls the function GetAll() in the.svc file like this:

public List<[Entity]> GetList()
{
    [iObject] repository = new [Object](new Context());
    return repository.GetAll().ToList();
}

The project is building. I even check the cconfiguration and it is in the right DB. However, database and tables which is supposedly created doesn't exist and an error "Sequence contains no matching element" shows up in the return.

If this one is confusing can you point me some link of WCF Services using Code First Entity Framework.

回答1:

I had a bug where I got a System.InvalidOperationException with the message Sequence contains no matching element because I had an incorrect string value being passed to .HasColumnType() in my table configuration.

回答2:

I just experienced the same problem. The Code First from database wizard generated one of the columns as [Column(TypeName = "date")].

Calling .saveChanges() resulted in the Sequence contains no elements.

After changing the defined column to [DataType(DataType.DateTime)] it worked as expected.

回答3:

I was also getting same error while adding migration or updating database.

The reason was I was using wrong column type

[Column("ImageType", TypeName = "varchar(20)")]
public string ImageType
{
     get;
     set;
}

But when I removed 20 from varchar it started working for me

[Column("ImageType", TypeName = "varchar")]
public string ImageType
{
    get;
    set;
}

回答4:

For me this message was because of misunderstanding for TypeName in Column attribute.

Column Attribute: [Column (string name, Properties:[Order = int],[TypeName = string])

name: Name of a column in a db table.
Order: Order of a column, starting with zero index. (Optional)
TypeName: Data type of a column. (Optional)

This TypeName must be only name of the type and must not include precision or scale or length and any other thing. For example following will cause an error

[Column(TypeName = "nvarchar(600)")]

while below one will work fine however you might want to have specific size of the column and for that one way is to use fluent API

[Column(TypeName = "nvarchar")]

回答5:

Entity Framework throws this exception if the column type is invalid. For example:

// This will throw an error. There is no such type name.
[Column(TypeName = "Invalid")]
public string Column1 { get; set; }

// Works.
[Column(TypeName = "varchar")]
public string Column1 { get; set; }

See these examples:

Example 1
Example 2
Other examples in comments.

回答6:

Yup. Caught me as well because in a moment of distraction I put the literal long in there for a bigint column i.e. HasColumnType("long")- can you believe it?! What a clown!

Generators generally create your EF classes properly, but if you're not able to or don't want to use one then you could use a static class with some static string fields on them so that you can intellisense it quite nicely:

public static class DatabaseColumnTypes
{
    /// <summary>
    /// Use this for 'boolean' values.
    /// </summary>
    public static string BitColumn = "bit";

    /// <summary>
    /// Use this for 'byte' values.
    /// </summary>
    public static string TinyIntColumn = "tinyint";

    /// <summary>
    /// Use this for 'long' values.
    /// </summary>
    public static string BigIntColumn = "bigint";

    /// <summary>
    /// Use this for 'string' values.
    /// </summary>
    public static string VarcharColumn = "varchar";

    // etc
}

Now you can do HasColumnType( DatabaseColumnTypes.BigIntColumn )

I know, I know, this is lazy, I should remember these types but every so often I find myself on Stack Overflow looking it up and this just saves time…

回答7:

I got the same error message when I had renamed my Member model to Student and I had a navigation property in some other class as:

public IList<Student> Members { get; set; }

I changed that to:

public IList<Student> Students { get; set; }

and the problem was resolved!

回答8:

I got this error while defining Table-Per-Type Inheritance with Fluent API (as a class deriving from EntityTypeConfiguration<T>) and mistakenly redefined field that was already defined in base class configuration (and both were exactly the same). I was using EntityFramework 6.2.0 nuget package.

回答9:

I got caught with this one because I referenced the reference property instead of the Id property.

        var t = modelBuilder.Entity<CyDocument>();
        t.HasIndex(b => b.DocTypeId);

回答10:

For me it helped, when switched class project to Set as startup project

来源：https://stackoverflow.com/questions/22982749/sequence-contains-no-matching-element-entityframework

标签

wcf

entity-framework

ef-code-first

wcf-data-services