I recently came across two cases where parameters are valid if passing by value, but result in compilation errors after adding “ref”. Consider the following member and method:

// SuperDataTable inherits from DataTable
// Indexing a SuperDataTable yields a SuperDataRow
// which inherits from DataRow.
private SuperDataTable mySuperDataTable = new SuperDataTable();

public void DoSomethingSuper(ref DataRow myDataRow)
{
	return;
}

Now examine the following three scenarios:

// #1 You receive a compiler error "A property or indexer
// may not be passed as an out or ref parameter"
DoSomethingSuper(ref mySuperDataTable[0]);

// #2 You receive a compiler error "Cannot convert from
// 'ref SuperDataRow' to 'ref DataRow'
SuperDataRow mySuperDataRow = mySuperDataTable[0];
DoSomethingSuper(ref mySuperDataRow);

// #3 Only does it work in the following manner, when you
// explicitly "upcast" the row to the exact type in the
// signature of the method
DataRow myDataRow = mySuperDataTable[0];
DoSomethingSuper(ref myDataRow);

Try it again, this time passing by value. No compiler errors! I probably am just lacking an intimate understanding of “ref”, but this came out of left field for me and I haven’t seen it documented online. Can anybody fill me in as to why scenarios #1 and #2 don’t work?