减或增十进制值

Question

in my c# solution i have :

private void DecrementProduct()
    {
        decimal? difference = this.Difference;

        this.Difference = new decimal?((difference.HasValue ?    difference.GetValueOrDefault() : new decimal(0))--);
    }

visual studio return error : Error 107 :The operand of an increment or decrement operator must be a variable, property or indexer.

where is problem?

Answer 1

The error is pretty self explanatory. You are using the decrement operator on a value, it needs to be a variable property or indexer.

It's akin to doing:

var foo = 10--;

You should probably just make it this:

this.Difference = difference - 1 ?? -1;

Answer 2

private void DecrementProduct()
{
    if(this.Difference.HasValue)
        this.Difference = this.Difference.Value - 1M;
    else
        this.Difference =  -1M;
}