Set default value to null when converting to double in c#

A double cannot be null since it's a value- and not a reference type. You could use a Nullable<double> instead:

double? ingredientMinRange = null;
if(!string.IsNullOrEmpty(MinRange))
    ingredientMinRange = Convert.ToDouble(MinRange);

If you later want the double value you can use the HasValue and Value properties:

if(ingredientMinRange.HasValue)
{
    double value = ingredientMinRange.Value;
}

Using Nullable Types (C# Programming Guide)

---

If IngredientMinRange is already a Double?-property as commented you can assign the value either via if(as shown above) or in in one line, but then you have to cast the null:

IngredientMinRange = string.IsNullOrEmpty(MinRange) ? (double?)null : Convert.ToDouble(MinRange);

to assign null to a double you have to use Nullable<double> or double?. Assign it with this method here:

decimal temp;
decimal? IngredientMinRange = decimal.TryParse(MinRange, out temp) ? temp : (decimal?)null;

then you can continue working with IngredientMinRange. You get the value with IngredientMinRange.Value or check if it's null with IngredientMinRange.HasValue