IF陈述式中的C＃OdbcDataReader.GetBoolean InvalidCastException

Question

Please forgive my broken English first.

Well,antiduh's method of debug helps me to know what happen on my code,thanks everyone who join discuss.

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When I use OdbcDataReader.GetBoolean single,it's OK and no problem,just like below:

OdbcDataReader reader = CMD.ExecuteReader();
reader.Read();
checkBox1.Checked = reader.GetBoolean(0);                

no problem wiil occur.

BUT!!

I need to ascertain the column is Null or not,so I use if-statement as below:

OdbcDataReader reader = CMD.ExecuteReader();
reader.Read();
if (reader.IsDBNull(0) == false)
    checkBox1.Checked = reader.GetBoolean(0);

It will occur InvalidCastException error on reader.GetBoolean(0)

I have no idea about this problem,does somebody can help me? please~

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According to antiduh's valuable comment, I found that if reader.GetValue(0) isn't in if-statement as below:

OdbcDataReader reader = CMD.ExecuteReader();
reader.Read();
var foo = reader.GetValue(0);

It wiil return True or False.

But if reader.GetValue(0) is in interior of if-statement as below:

OdbcDataReader reader = CMD.ExecuteReader();
reader.Read();
if (reader.IsDBNull(0) == false)
{
    var foo = reader.GetValue(0);
}

It will return 1 or 0.

So if I use GetBoolean to transform 1 or 0 to bool data type,will occur InvalidCastException.

Now I know what happen on my code,but still don't know what the difference between two codes.

---

Thanks antiduh's valuable comment again,

I try this code that antiduh provide below:

OdbcDataReader reader = CMD.ExecuteReader();
reader.Read();
if (reader.IsDBNull(0) == false)
{
    try
    {
        checkBox1.Checked = reader.GetBoolean(0);
    }
    catch ( InvalidCastException e ) {
        object doubleCheck = reader.GetValue(0);
        Console.WriteLine( "Tried to cast this type: " + doubleCheck.GetType() );
    }
}

It wiil return: Tried to cast this type:System.String

And doubleCheck.ToString() is 1 or 0.

Answer 1

Its likely that the value in that row and column is not actually a bool. Add some temporary debugging code to catch the exception and call GetValue instead, and look to see what you're actually getting back.

OdbcDataReader reader = CMD.ExecuteReader();
reader.Read();
if (reader.IsDBNull(0) == false)
{
    try {
        checkBox1.Checked = reader.GetBoolean(0);
    }
    catch ( InvalidCastException e ) {
        object doubleCheck = reader.GetValue(0);

        Console.WriteLine( "Tried to cast this type: " + doubleCheck.GetType() );
    }
}