TryGetValue in C# doesn't work on strings, does it?

Question

The object Row is a class, that has a property Values which is a Dictionary.

Below are extension methods on the Values property.

public static T TryGetValue<T>(this Row row, string key)
{
return TryGetValue(row, key, default(T));
}

public static T TryGetValue<T>(this Row row, string key, T defaultValue)
{
    object objValue;

    if (row.Values.TryGetValue(key, out objValue))
    {
        return (T)objValue;
    }

    return defaultValue;
}

If I do:

user.Username = user.Values.TryGetValue<string>("Username");

This happends if the key "username" is not in the Dictionary.

I get an exception, invalid cast:

The following error ocurred:

System.InvalidCastException: Specified cast is not valid.

TryGetValue[T](Row row, String key, T defaultValue) 

TryGetValue[T](Row row, String key) 

So I guess TryGetValue doesn't work on strings?

Answer 1

It should work fine, either if the key "Username" is in the dictionary with a corresponding string value, or not in the dictionary at all.

The fact that you're getting an InvalidCastException shows that the value for the "Username" key wasn't a string.

Answer 2

Is it possible you've got an entry in your dictionary with key "Username" whose value is not a string?

I've added comments to your method explaining how this could lead to your issue.

// I'm going to go ahead and assume your Values property
// is a Dictionary<string, object>
public static T TryGetValue<T>(this Row row, string key, T defaultValue)
{
    // objValue is declared as object, which is fine
    object objValue;

    // this is legal, since Values is a Dictionary<string, object>;
    // however, if TryGetValue returns true, it does not follow
    // that the value retrieved is necessarily of type T (string) --
    // it could be any object, including null
    if (row.Values.TryGetValue(key, out objValue))
    {
        // e.g., suppose row.Values contains the following key/value pair:
        // "Username", 10
        //
        // then what you are attempting here is (string)int,
        // which throws an InvalidCastException
        return (T)objValue;
    }

    return defaultValue;
}