在C＃中使用类型作为函数参数的通用对象创建

Question

I'm trying to create a generic function to parse my json result with Newtonsoft:

private T ParseResult<T>(string queryResult)
{
    Result res = JsonConvert.DeserializeObject<Result>(queryResult);

    if (res.Success == 1)
    {
        try
        {
            return JsonConvert.DeserializeObject<T>(JsonConvert.SerializeObject(res.data));
        }
        catch (Exception)
        {
            return default(T);
        }
    }
    return default(T);
}

If there is a problem with Success or the parsing I want to return an empty object of whatever T is (currently lists or just custom objects).

My problem is that the current solution is returning null instead of an empty object. How can I achieve that the return value will never be null.

Answer 1

Irregardless of whether this is the right or wrong approach. The problem is default(T) will return the default for the type , for Reference Types that is null . If you want an "empty object" (new object) then you will have to use the new constraint and new it up (instantiate it)

Example

private T ParseResult<T>(string queryResult) where T : new()
{

    ...

    return new T();

}

Note : There are caveats though

new constraint (C# Reference)

The new constraint specifies that any type argument in a generic class declaration must have a public parameterless constructor . To use the new constraint, the type cannot be abstract.

---

Additional Resources

default value expressions (C# programming guide)

A default value expression default(T) produces the default value of a type T . The following table shows which values are produced for various types:

• Any reference type : null
• Numeric value type : 0
• bool : false
• char : \\0
• enum : The value produced by the expression (E)0 , where E is the enum identifier.
• struct : The value produced by setting all value type fields to their default value and all reference type fields to null .
• Nullable type : An instance for which the HasValue property is false and the Value property is undefined.

Answer 2

T could be a class that does not have a default constructor: in this case new T() would be an invalid statement.

If you want to return with the default object you can use Activator.CreateInstance instead of returning default(T) as below:

return Activator.CreateInstance(typeof(T));

if you want to pass parameters in it, then use as below:

return (T)Activator.CreateInstance(typeof(T), args);

Answer 3

Thanks for the responses. I ended up creating this function which seems to work and I think is a better approach:

    private static T ParseResult<T>(string queryResult) where T : new()
    {
        try
        {
            Result<T> res = JsonConvert.DeserializeObject<Result<T>>(queryResult);

            if (res.Success == 1)
            {
                return res.Data;
            }
            return new T();
        }
        catch (Exception) {
            return new T();
        }
    }

and

internal class Result<T>
{
    [JsonProperty("success")]
    public int Success { get; set; }
    [JsonProperty("data")]
    public T Data { get; set; }
}