检查多个子对象为null的最佳方法

Question

I would like to test for null to avoid exception. I receive list of objects which i have to export to excel and display in Grid.Mvc

Excel export:

sheet.Cells["A" + i].Value = item.Car.Door.Code; //FLCo

Grid display:

columns.Add(c => c.Car.Door.Code).Titled("FLCo");

The thing is Car can be null, Door can be null.

Q1: For the excel export, my solution is to use a couple of if/else (any better way)?

For the Grid display: if/else or "?" operator is not supported inside the linq

the following will generate error

columns.Add(c => c.Car==null?"":(c.Car.Door==null?"":c.Car.Code)).Titled("FLCo");

error:

Cannot convert lambda expression to type 'GridMvc.Columns.IGridColumn' because it is not a delegate

Q2: Any idea how to solve this?

Answer 1

If you're using C#6 (which is included in VS2015, thanks HimBromBeere.), you can write it as followed:

sheet.Cells["A" + i].Value = item?.Car?.Door?.Code;

If any of the properties is NULL, the result will be NULL.

Answer 2

As for Q2 : You can use statement lambda s by enclosing the statement in curly braces: https://msdn.microsoft.com/de-de/library/bb397687.aspx#Anchor_1

So in your case it would be

columns.Add(c => {c.Car==null?"":(c.Car.Door==null?"":c.Car.Code)}).Titled("FLCo");

Answer 3

You can use "Elvis operator" . Or if not supported I prefer to use extension methods.

public static class Maybe
{
    public static TResult With<TInput, TResult>
        (this TInput o, Func<TInput, TResult> evaluetor)
        where TInput : class
        where TResult : class
    {
        return o == null ? null : evaluetor(o);
    }

    public static TResult Return<TInput, TResult>
        (this TInput o, Func<TInput, TResult> evaluator, TResult failureValue)
        where TInput : class
    {
        return o == null ? failureValue : evaluator(o);
    }
}

So in code you can just do something like

sheet.Cells["A" + i].Value = item.With(x => x.Car).With(x => x.Door).Return(x => x.Code, "null");