Avoid multiple similar foreach - C#

Delegates come in very handy in many cases and definitely in such a case. I know this has already been answered and rightly so, but here is an alternative for comparison. I have provided a link to give you some insight.

public class CalculationMethods
{
    public delegate void CalculationDelegate(Detail method);

    private Dictionary<string, CalculationDelegate> _methods;

    public CalculationMethods
    {
        this._methods = new Dictionary<string, CalculationDelegate>()
        {
            { "Discount", CalculateDiscount },
            { "Tax",      CalculateTax      }
        };
    }

    public void Calculate(string method, Detail obj_detail)
    {
        foreach (Order order in ordersList)
        {
            foreach (Detail obj_detail in order.Details)
            {
                var m = this._methods.FirstOrDefault(item => item.Key == method).Value;
                m(obj_detail);
            }
        }
    }
}

Usage:

//Initialize
var methods = new CalculationMethods();

//Calculate Discount
methods.Calculate("Discount", obj_detail);

//Calculate Tax
methods.Calculate("Tax", obj_detail);

Side Note: I would recommend some exception handling in case the method of calculation isn't found among the list of delegates. Example below: (Replace the calculate method with the following.)

public void Calculate(string method, Detail obj_detail)
{
    foreach (Order order in ordersList)
    {
        foreach (Detail obj_detail in order.Details)
        {
            var m = this._methods.FirstOrDefault(item => item.Key == method).Value;

            //Check if the method was found
            if (m == null)
                throw new ApplicationNullException("CalculationDelegate")

            m(obj_detail);
        }
    }
}

Decent tutorial: Delegates and Events

You can use delegates. (Google it - I don't have a development environment in front of me to run up a sample for you). Basically one method that takes a delegate to call: Here is pseudo code...

public void CalculationMethod(delegate myFunction) // need to look up correct param syntax
{
    foreach (Order order in ordersList)
    {
        foreach (Detail obj_detail in order.Details)
        {
            myFunction(); // Need to lookup correct calling syntax
        }
    }
}

I googled "c# delegate as parameter" and came up with http://msdn.microsoft.com/en-us/library/ms173172.aspx which seems to be a reasonable explanation.

As Darren Kopp says, you can use delegates. However, in the case that you are calling a method with a parameter, you can call it directly (you don't need the lambda expression).

With public void ApplyToDetails(Action<Detail> callback) { ... }:

ApplyToDetails(Method_1);         // Uses objDetail
ApplyToDetails(d => Method_2());  // Doesn't use objDetail
ApplyToDetails(d => Method_3());  // Doesn't use objDetail
ApplyToDetails(Method_4);         // Uses objDetail

Note that you must not place parameter braces after the methods you pass as delegate!

You could use delegates as the other answers have provided but I believe in your case doing so will lead to overly confusing code. Your code is cleaner and more readable if you redeclare the foreach loops in each method. Only if you were copy-pasting portions of the internals would I say you run the risk of code duplication.

Think about it this way: If you created a method passing in a delegate, what would the name of this method be called? It is a method that does something for each Detail in each Order you pass in and should be named something like DoSomethingForEachDetailInOrders(). What kind of class would this method exist for? You don't know what it is you're actually doing in the delegate, so the purpose of this class would have to be more framework-style code, which your app does not appear to be complex enough to warrant.

Additionally, if you were debugging this code or reading through it, instead of being able to see 2 foreach loops inside the method you are reading, you have to go scroll to the definition of the delegate, read that, and then go back to your method and resume reading.

Edit: I originally answered this question by downplaying the duplication of the foreach loops in the hopes that OP would not add additional complexity to his app attempting to make it follow "best practices." I didn't go deeper because the code requires a more intrusive refactor for maintainability. The foreach loop code smell stems from other problems as detailed in the comments below this answer. I still stand by my opinion that adding the delegate method is less desirable than the duplicated loops because the delegate method option is pretty much textbook boilerplate.

Added a code example to explain how the code should be refactored if maintainability is a concern:

public decimal CalculateDiscount(IEnumerable<Order> ordersList)
{
    return ordersList.SelectMany(order => order.Details).Sum(detail => detail.Discount);
}

public decimal CalculateTax(IEnumerable<Order> ordersList)
{
    return ordersList.SelectMany(order => order.Details).Sum(detail => detail.Total) * taxRate;
}

If you ABSOLUTELY MUST HAVE a custom function for getting all details for orders (could be refactored to an extension method):

public IEnumerable<Detail> GetDetailsForOrders(IEnumerable<Orders> orderList)
{
    foreach(var order in orderList)
    {
        foreach (var detail in order.Details)
        {
            yield return detail;
        }
    }
}

public decimal CalculateDiscount(IEnumerable<Order> ordersList)
{
    return GetDetailsForOrders(ordersList).Sum(detail => detail.Discount);
}

public decimal CalculateTax(IEnumerable<Order> ordersList)
{
    return GetDetailsForOrders(ordersList).Sum(detail => detail.Total) * taxRate;
}