Delete all entities in Entity Framework

This will perform much, much better than anything involving deleting individual entity objects, assuming the underlying database is MSSQL.

foreach (var tableName in listOfTableNames)
{
    context.ExecuteStoreCommand("TRUNCATE TABLE [" + tableName + "]");
}

Of course, if your tables have foreign-key relationships, you'll need to set up your list of table names in the proper order so that you clear foreign-key tables before you clear any primary-key tables that they might depend upon.

Just for lazy ones, code I came up myself when looking for the answer:

public static void ClearDatabase<T>() where T : DbContext, new()
{
    using (var context = new T())
    {
        var tableNames = context.Database.SqlQuery<string>("SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_TYPE = 'BASE TABLE' AND TABLE_NAME NOT LIKE '%Migration%'").ToList();
        foreach (var tableName in tableNames)
        {
            context.Database.ExecuteSqlCommand(string.Format("DELETE FROM {0}", tableName));
        }

        context.SaveChanges();
    }
}

Short explanation: I do not truncate tables due to lack of permissions, if it’s not a problem for you, feel free to do so. The table __MigrationHistory is ignored by the where statement.

UPDATE: After some research I came up with better solution (not as nice but deletes only required columns):

public static void ClearDatabase(DbContext context)
{
    var objectContext = ((IObjectContextAdapter)context).ObjectContext;
    var entities = objectContext.MetadataWorkspace.GetEntityContainer(objectContext.DefaultContainerName, DataSpace.CSpace).BaseEntitySets;
    var method = objectContext.GetType().GetMethods().First(x => x.Name == "CreateObjectSet");
    var objectSets = entities.Select(x => method.MakeGenericMethod(Type.GetType(x.ElementType.FullName))).Select(x => x.Invoke(objectContext, null));
    var tableNames = objectSets.Select(objectSet => (objectSet.GetType().GetProperty("EntitySet").GetValue(objectSet, null) as EntitySet).Name).ToList();

    foreach (var tableName in tableNames)
    {
        context.Database.ExecuteSqlCommand(string.Format("DELETE FROM {0}", tableName));
    }

    context.SaveChanges();
}

For EF 6:

DbSet<Entity>.RemoveRange(DbSet<Entity>);