How to clean-up an Entity Framework object context?

I am adding several entities to an object context.

try
{
    forach (var document in documents)
    {
        this.Validate(document); // May throw a ValidationException.

        this.objectContext.AddToDocuments(document);
    }

    this.objectContext.SaveChanges();
}
catch
{
    // How to clean-up the object context here?

    throw;
}

If some of the documents pass the the validation and one fails, all documents that passed the validation remain added to the object context. I have to clean-up the object context because it may be reused and the following can happen.

var documentA = new Document { Id = 1, Data = "ValidData" };
var documentB = new Document { Id = 2, Data = "InvalidData" };
var documentC = new Document { Id = 3, Data = "ValidData" };

try
{
    // Adding document B will cause a ValidationException but only
    // after document A is added to the object context.
    this.DocumentStore.AddDocuments(new[] { documentA, documentB, documentC });
}
catch (ValidationException)
{
}

// Try again without the invalid document B. This causes an exception because
// of a duplicate primary key - document A with id 1 is added a second time.
this.DocumentStore.AddDocuments(new[] { documentA, documentC });

This will again add document A to the object context and in consequence SaveChanges() will throw an exception because of a duplicate primary key.

So I have to remove all already added documents in the case of an validation error. I could of course perform the validation first and only add all documents after they have been successfully validated but sadly this does not solve the whole problem - if SaveChanges() fails, all documents still remain added but unsaved.

I tried to detach all objects returned by

 this.objectContext.ObjectStateManager.GetObjectStateEntries(EntityState.Added)

but I am getting a exception stating that the object is not attached. So how do I get rid of all added but unsaved objects?

Daniel's answer worked for me, however the EntityFramework API is different in version 6+. Here is a method I added to my custom repository container that will detach all entities from the DbContext's ChangeTracker:

    /// <summary>
    /// Detaches all of the DbEntityEntry objects that have been added to the ChangeTracker.
    /// </summary>
    public void DetachAll() {

        foreach (DbEntityEntry dbEntityEntry in this.Context.ChangeTracker.Entries().ToArray()) {

            if (dbEntityEntry.Entity != null) {
                dbEntityEntry.State = EntityState.Detached;
            }
        }
    }

It was just a trivial bug but I am going to leave the question here - maybe it helps others.

I had the following

var objectStateEntries = this.objectContext
                             .ObjectStateManager
                             .GetObjectStateEntries(EntityState.Added);

foreach (var objectStateEntry in objectStateEntries)
{
    this.objectContext.Detach(objectStateEntry);
}

while I wanted the following

foreach (var objectStateEntry in objectStateEntries)
{
    this.objectContext.Detach(objectStateEntry.Entity);
}

and couldn't see it.

Since EF 5.0 use ChangeTracker.Clear() which clears the DbContext of all tracked entities.