Intercept all EF6 Linq queries

Question

I have a function which I want to run on every executed Linq query on a DbContext to modify the expression tree before execution. I've been having a look at the IDbCommandTreeInterceptor interface but that doesn't seem to provide an expression tree (which I suppose is understandable since it may not have been a Linq query by the time it gets to this point).

Is there any way I can intercept and modify all expressions before execution?

nb. This has to be Linq tree modification because I have already built a framework for modifying Linq trees which was originally for Linq to SQL.

Answer 1

Creating a proxy for the LINQ provider to intercept every LINQ expression execution (as suggested in the comments) is still a good solution. In fact, I'm playing around with this stuff within this project, which explicitly supports EF6 including EF6 async queries. You can create a standard .NET ExpressionVisitor to make an interception:

intercepted = query.Rewrite(new MyInterceptor());

But the question also demands "to run on every executed Linq query on a DbContext", and that will be the tricky part. One approach can be some kind of abstraction of DbContext / DbSet, so your code does not directly access DbSet objects. And inside the implementation of this abstraction the interception can happen...

Another approach (and I think that answers this question best) would be a proxy for DbSet, which calls the LINQ proxy for queries, which enables interception. First, we have to inherit from DbSet:

public class DbSetProxy<TEntity> : DbSet<TEntity>,
                                   IQueryable<TEntity>,
                                   IDbAsyncEnumerable<TEntity>
    where TEntity : class
{
    private readonly DbSet<TEntity> set;
    private readonly DbQuery<TEntity> query;

    private readonly IQueryable<TEntity> intercepted;

    public DbSetProxy(DbSet<TEntity> set)
        : this(set, set)
    {
    }

    public DbSetProxy(DbSet<TEntity> set, DbQuery<TEntity> query)
    {
        this.set = set;
        this.query = query;

        // use NeinLinq or any other LINQ proxy library
        intercepted = query.Rewrite(new MyInterceptor());
    }
}

Then, it's necessary to overwrite all the members to call the actual DbSet for non query stuff:

(Note: It's unfortunately necessary to overwrite every DbSet member, because inheriting DbSet is only designed for test stubs. Thus, just inheriting DbSet breaks the DbSet.)

public override DbQuery<TEntity> AsNoTracking()
{
    return new DbSetProxy<TEntity>(set, query.AsNoTracking());
}

public override DbQuery<TEntity> AsStreaming()
{
    return new DbSetProxy<TEntity>(set, query.AsStreaming());
}

public override DbQuery<TEntity> Include(string path)
{
    return new DbSetProxy<TEntity>(set, query.Include(path));
}

public override TEntity Add(TEntity entity)
{
    return set.Add(entity);
}

public override IEnumerable<TEntity> AddRange(IEnumerable<TEntity> entities)
{
    return set.AddRange(entities);
}

public override TEntity Attach(TEntity entity)
{
    return set.Attach(entity);
}

public override TEntity Create()
{
    return set.Create();
}

public override TDerivedEntity Create<TDerivedEntity>()
{
    return set.Create<TDerivedEntity>();
}

public override TEntity Find(params object[] keyValues)
{
    return set.Find(keyValues);
}

public override Task<TEntity> FindAsync(params object[] keyValues)
{
    return set.FindAsync(keyValues);
}

public override Task<TEntity> FindAsync(CancellationToken cancellationToken, params object[] keyValues)
{
    return set.FindAsync(cancellationToken, keyValues);
}

public override TEntity Remove(TEntity entity)
{
    return set.Remove(entity);
}

public override IEnumerable<TEntity> RemoveRange(IEnumerable<TEntity> entities)
{
    return set.RemoveRange(entities);
}

public override DbSqlQuery<TEntity> SqlQuery(string sql, params object[] parameters)
{
    return set.SqlQuery(sql, parameters);
}

public override ObservableCollection<TEntity> Local
{
    get { return set.Local; }
}

public override bool Equals(object obj)
{
    return set.Equals(obj);
}

public override int GetHashCode()
{
    return set.GetHashCode();
}

public override string ToString()
{
    return set.ToString();
}

Finally, we've to make use of the interception object:

IEnumerator<TEntity> IEnumerable<TEntity>.GetEnumerator()
{
    return intercepted.GetEnumerator();
}

IEnumerator IEnumerable.GetEnumerator()
{
    return intercepted.GetEnumerator();
}

Type IQueryable.ElementType
{
    get { return intercepted.ElementType; }
}

Expression IQueryable.Expression
{
    get { return intercepted.Expression; }
}

IQueryProvider IQueryable.Provider
{
    get { return intercepted.Provider; }
}

IDbAsyncEnumerator<TEntity> IDbAsyncEnumerable<TEntity>.GetAsyncEnumerator()
{
    return ((IDbAsyncEnumerable<TEntity>)intercepted).GetAsyncEnumerator();
}

IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator()
{
    return ((IDbAsyncEnumerable<TEntity>)intercepted).GetAsyncEnumerator();
}

And, at last, we can use an ordinary DbContext. We only have to overwrite its Set method to inject our proxy:

public class MyContext : DbContext
{
    public DbSet<Entity> Entities { get; set; }

    public override DbSet<TEntity> Set<TEntity>()
    {
        return new DbSetProxy<TEntity>(base.Set<TEntity>());
    }
}