Convert.Changetype() Fails on Nullable Types

Convert.ChangeType() fails on Nullable Types

Untested, but maybe something like this will work:

string modelProperty = "Some Property Name";
string value = "Some Value";

var property = entity.GetType().GetProperty(modelProperty);
if (property != null)
{
    Type t = Nullable.GetUnderlyingType(property.PropertyType) ?? property.PropertyType;

    object safeValue = (value == null) ? null : Convert.ChangeType(value, t);

    property.SetValue(entity, safeValue, null);
}

Convert.ChangeType throwing Invalid Cast Exception for nullable int

I'm assuming Claim.Value is of type Object and you're dynamically converting here, you can't straight up convert an int to an int? via Convert.ChangeType.

One option is to use Nullable.GetUnderlyingType which will check if this is a nullable struct case, do the conversion via the underlying data type first, then cast to T.

You'll also need to handle the null scenario as well.

if (claim != null)
{
    var conversionType = typeof(T);

    if (Nullable.GetUnderlyingType(conversionType) != null)
    {
        if (claim.Value == null) //check the null case!
            return default(T);

        //use conversion to `int` instead if `int?`
        conversionType = Nullable.GetUnderlyingType(conversionType);
    }

    return (T)Convert.ChangeType(claim.Value, conversionType);
}

How to use Convert.ChangeType() when conversionType is a nullable int

You can't do this since Nullable<T> don't implement IConvertable.

You can do this although.

string a = 24;
decimal? aAsDecimal = (decimal)Convert.ChangeType(a, typeof(decimal));

Might I also interest you in TryParse?

Unable to ChangeType from string to int?

You'll need to write something like this:

var value = "00010";
short? w = value == null ? null : (short?)Convert.ChangeType("00010", typeof(short));

ChangeType will not work with Nullable<T> - and if we have a value for value, we assume we have a value for the converted type

How to Convert a String to a Nullable Type Which is Determined at Runtime?

There are two problems.

Firstly, Convert.ChangeType just plain does not support nullable types.

Secondly, even if it did, by boxing the result (assigning it to an object), you'd already be converting it to a DateTime.

You could special case nullable types:

string s = "2012-02-23 10:00:00";
Type t = Type.GetType("System.Nullable`1[[System.DateTime, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]");
object d;

if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>))
{
    if (String.IsNullOrEmpty(s))
        d = null;
    else
        d = Convert.ChangeType(s, t.GetGenericArguments()[0]);
}
else
{
    d = Convert.ChangeType(s, t);
}

How can I fix this up to do generic conversion to NullableT?

public static T To<T>(this IConvertible obj)
{
    Type t = typeof(T);
    Type u = Nullable.GetUnderlyingType(t);

    if (u != null)
    {
        return (obj == null) ? default(T) : (T)Convert.ChangeType(obj, u);
    }
    else
    {
        return (T)Convert.ChangeType(obj, t);
    }
}

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