How to query EntityFramework using generic property type?

Question

When I call DbSet.FirstOrDefault() by passing a predicate that compares generic type TId, I get the following exception:

unable to create a constant value of type 'system.object'. only primitive types or enumeration types are supported in this context.

Interface for type being queried:

interface IEntity<TId>
{
    TId id { get; set; }
}

The exception is thrown here:

public virtual TEntity Get<TEntity, TId>(TId id) where TEntity : class, IEntity<TId>
{
    return dbContext.Set<TEntity>().FirstOrDefault(e => e.Id.Equals(id));
}

The function will only work if TId is constrained as struct. How can I include string as supported type? If it's not possible, is it possible to accomplish the task a different way?

UserControl · Accepted Answer · 2017-02-02 06:31:51Z

2

This will work for strings too:

public virtual TEntity Get<TEntity, TId>(TId id) 
    where TEntity : class, IEntity<TId>
    where TId: IEquatable<TId>
{
    return dbContext.Set<TEntity>().FirstOrDefault(e => e.Id.Equals(id));
}

answered Feb 2, 2017 at 6:31

UserControl

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3 Comments

user845279 Over a year ago

How can i extend it to work for arrays of these primitives too? Ex: int[] and string[]. Thanks.

UserControl Over a year ago

I don't think you can. Equality for sets (RDBMSs have no concept of arrays) is too broad and there is no direct ==-like support for it in SQL. Consider for example [1,2] and [2,1]. Are they equal to you? Is ordering important? What about [null, 1] and [null,1]? Do you care about ternary logic here or not, etc.

user845279 Over a year ago

I'm working with MS Sql which has Binary[] type. It gets converted to System.Byte[].

Slava Utesinov · Accepted Answer · 2017-02-02 07:22:57Z

2

You can simply use Find method:

public virtual TEntity Get<TEntity, TId>(TId id) where TEntity : class, IEntity<TId>
{
    return dbContext.Set<TEntity>().Find(id);
}

answered Feb 2, 2017 at 7:22

Slava Utesinov

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3 Comments

user845279 Over a year ago

I tried that and it works, but it fails for cases where the primary key is a composite key.

Slava Utesinov Over a year ago

It is strange, how did you try it? dbContext.Set<TEntity>().Find(id1, id2);?

user845279 Over a year ago

Just as you mentioned in your post. It's just that, using Find(), you can't choose to use just one of the keys.

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