Generic Data Access Helper using Entity Framework 6.1 (Code First)-优快云博客

原文：http://www.codeproject.com/Articles/898206/Generic-Data-Access-Helper-using-Entity-Framework?msg=5044112#xx5044112xx

Motivation

When we write a simple logic that accesses database using EF & Linq, it simply looks like this:

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public static List<Employee> GeAllEmployees()
{
    try
    {
        using (var northwindContext = new NorthwindDBContext())
        {
            var query = from e in northwindContext.Employees select e;
            return query.ToList();
        }
    }
    catch (Exception ex)
    {
        // Log Error
    }
}

So, I have to make this code redundant with a new business (e.g. GetEmployeeOrders). Also, in case I have to access another database which means another DBContext, I have to make this logic redundant !!

Here, the Generics and Delegates come as a solution for those two issues. So I created a public static class called DALHelper containing the following seven static methods.

1 Querying

All retrieval's methods could also be used for saving changes to the database.

1.1 Default

The following snippet code is locking tables and this is the default behavior of initiating a new DbContext.

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public static bool GenericRetrival<T>(Action<T> action) where T : DbContext, new()
{
    try
    {
        using (var context = new T())
        {
            action(context);
            return true;
        }
    }
    catch (Exception ex)
    {
	// Log Error
        return false;
    }
}

The usage

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public List<Employee> GeAllEmployees()
{
	List<Employee> result= null;
	bool success = DALHelper.GenericRetrival<NorthwindDBContext>((northwindContext) =>
	{
		result = (from e in northwindContext.Employees select e).ToList();
	});
	return result;
}

1.2 Querying with a generic result

Here we identify TResult as generics of type DBContext which a delegate of type Func is going to return an object of TResult type.

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public static TResult GenericResultRetrival<T, TResult>(Func<T, TResult> func) where T : DbContext, new()
    where TResult : new()
{
    try
    {
        using (var context = new T())
        {
            TResult res = func(context);
            return res;
        }
    }
    catch (Exception ex)
    {
        // Log Error
        return default(TResult);
    }
}

The usage

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public List<Employee> GeAllEmployees()
{
	List<Employee> result = DALHelper.GenericResultRetrival<NorthwindDBContext,List<Employee>>((northwindContext) =>
	{
		return (from e in northwindContext.Employees select e).ToList();
	});
	return result;
}

1.3 Querying asynchronously

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public static async Task<TResult> GenericRetrivalAsync<T, 
	TResult>(Func<T, Task<TResult>> func)
    where T : DbContext, new()
    where TResult : new()
{
    try
    {
        using (var context = new T())
        {
            return await func(context);
        }
    }
    catch (Exception ex)
    {
	// Log Error
        return default(TResult);
    }
}

The usage

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public async Task<List<Employee>> GetAllEmployeesAsync()
{
    return await DALHelper.GenericRetrivalAsync<NorthwindDBContext, List<Employee>>(async (northwindContext) =>
    {
        return await (from e in northwindContext.Employees select e).ToListAsync();
    });
}

1.4 A long query with no locking tables asynchronously

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public static async Task<TResult> 
GenericResultNoLockLongRetrivalAsync<T,TResult>(Func<T, Task<TResult>> func)
    where T : DbContext, new()
    where TResult : new()
{
    try
    {
        using (var context = new T())
        {
            ((IObjectContextAdapter)context).ObjectContext.CommandTimeout = 0;
            using (var dbContextTransaction = 
            	context.Database.BeginTransaction(IsolationLevel.ReadUncommitted))
            {
                return await func(context);
            }
        }
    }
    catch (Exception exception)
    {
        // Log Error
        return default(TResult);
    }
}

1.5 Querying from twice contexts asynchronously

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public static async Task<object> 
	GenericTwiceContextsRetrivalAsync<T1, T2>(Func<T1, T2, Task<object>> func)
            where T1 : DbContext, new()
            where T2 : DbContext, new()
{
    try
    {
        using (var context1 = new T1())
        {
            using (
                var dbContextTransaction1 = context1.Database.BeginTransaction(IsolationLevel.ReadUncommitted))
            {
                using (var context2 = new T2())
                {
                    using (
                        var dbContextTransaction2 =
                            context2.Database.BeginTransaction(IsolationLevel.ReadUncommitted)
                        )
                    {
                        return await func(context1, context2);
                    }
                }
            }
        }
    }
    catch (Exception exception)
    {
        // Log Error

        return null;
    }
}

The usage

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public async Task<object> GetDistributedDataAsync()
{
    return await DALHelper.GenericTwiceContextsRetrivalAsync<NorthwindDBContext, AdventureWorkDBContext>(async
        (northwindContext, advantureContext) =>
        {
            var employees = (from e in northwindContext.Employees select e).ToListAsync();
            var cutomers = (from c in advantureContext.Customers select c).ToListAsync();

            await Task.WhenAll(employees, cutomers);
            return new
            {
                EmployeeList = employees.Result,
                PersonList = cutomers.Result
            };
        });
}

So the design is going to be:

2 Saving

2.1 Generic safe saving

I called it safe as it could treat a set of transactions as an atom with commit / rollback logic.

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public static bool GenericSafeTransaction<T>(Action<T> action) where T : DbContext, new()
{
    using (var context = new T())
    {
        using (var dbContextTransaction = context.Database.BeginTransaction())
        {
            try
            {
                action(context);
                dbContextTransaction.Commit();
                return true;
            }
            catch (Exception ex)
            {
                dbContextTransaction.Rollback();
                // Log Error
                return false;
            }
        }
    }
}

The usage

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public bool AddMultipleRecords(Employee newEmp, Supplier newSup)
{
    return DALHelper.GenericSafeTransaction<NorthwindDBContextgt;(northwindContext =>
    {
        northwindContext.Employees.Add(newEmp);
        northwindContext.SaveChanges();
        northwindContext.Suppliers.Add(newSup);
        northwindContext.SaveChanges();
    });
}

2.2 Saving asynchronously

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public static async Task<int?> GenericSafeTransactionAsync<T>(Action<T> action)
            where T : DbContext, new()
{
    using (var context = new T())
    {
        using (var dbContextTransaction = context.Database.BeginTransaction())
        {
            try
            {
                action(context);
                int affectedRecords = await context.SaveChangesAsync();
                dbContextTransaction.Commit();
                return affectedRecords;
            }
            catch (Exception ex)
            {
                dbContextTransaction.Rollback();
		// Log Error
                return null;
            }
        }
    }
}

The usage

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return await DALHelper.GenericSafeTransactionAsync<NorthwindDBContext>( async (northwindContext) =>
{
	northwindContext.Employees.Add(newEmp);
});