Intercept all EF6 Linq queries - linq

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>());
}
}