指定的类型转换无效-浮点数列表的双精度数列表

Question

So I store a list of float in a JSON file, this is what the JSON list looks like:

"RollSize": "[17.5,18.0,19.0,23.5,26.5,35.0,35.5,38.0]"

I use a method which returns a list of objects as there are multiple lists to return. I then cast the list of objects to a float. However the Specified cast is not valid receive an exception when doing this. But if I cast the list of objects to a double it works. Here is the two methods:

private void DisplayCutOffs(object sender, EventArgs e) {
            try {
// Errors here unless I cast to double 
                _view.CurrentCutOffValues = _systemVariablesManager.ReturnListBoxValues("CutOff").Cast<float>().ToList();
            }
            catch (Exception ex) {
                LogErrorToView(this, new ErrorEventArgs(ex.Message));
            }
        }

Repository method:

 public List<object> ReturnListBoxValues(string propertyName) {
            if (File.Exists(expectedFilePath)) {
                var currentJsonInFile = JObject.Parse(File.ReadAllText(expectedFilePath));
                return JsonConvert.DeserializeObject<List<object>>(currentJsonInFile["SystemVariables"][propertyName].ToString());
            }
            else {
                throw new Exception("Setup file not located. Please run the Inital Set up application. Please ask Andrew for more information.");
            }
        }

However I noticed that if I looped around the list in a foreach I could cast each value to a float. So I'm not sure what's going on here.

Anyone know?

Answer 1

It sounds like you're cast is casting from object to (the type), where (the type) is either float or double . This is an unboxing operation, and must be done to the right type . The value, as object , knows what it is - and this exception is thrown if you unbox it incorrectly (caveat: there is a little wriggle room if you unbox it to something the same size and compatible - for example you can unbox an int-enum to an int and vv).

Options:

• stick with object , but know what the data is and unbox it correctly - perhaps casting after the unbox, ie float f = (float)(double)obj; (the (double) here is the unbox from object to double ; the (float) is the type conversion from double to float )
• test the object type / use Convert.ToSingle
• change the property to be a defined type in the first place, rather than object

Full examples:

List<object> actuallyDoubles = new List<object>{ 1.0, 2.0, 3.0 };
List<double> doubleDirect = actuallyDoubles.ConvertAll(x => (double)x); // works
// List<float> floatDirect = actuallyDoubles.ConvertAll(x => (float)x); // fails per question
List<float> floatViaDouble = actuallyDoubles.ConvertAll(x => (float)(double)x); // works
List<float> floatViaConvert = actuallyDoubles.ConvertAll(x => Convert.ToSingle(x)); // works