SSRS Formula Expression - reporting-services

Question

can somebody help me make my SSRS Formula the same with my crystal report expression?
Crystal Formula is correct and working as I wanted to but SSRS is giving me wrong result.
Crystal Formula:
if IsNull({OrderDtl.XPartNum}) then {OrderDtl.PartNum}
else ( if IsNull({OrderHed.Calc_CustPartOpts}) then {OrderDtl.PartNum}
else ( select {OrderHed.Calc_CustPartOpts}
case "M": {OrderDtl.XPartNum}
case "S": {OrderDtl.PartNum}
case "O": {OrderDtl.XPartNum}
case "N": {OrderDtl.PartNum}
default: {OrderDtl.PartNum}
)
)
This is the result of the report using CR.
enter image description here
SSRS Formula:
=iif(Fields!XPartNum.Value="", "",
iif(isnothing(Fields!Calc_CustPartOpts.Value), "",
(Switch(UCase(Fields!Calc_CustPartOpts.Value) = "M", Fields!PartNum.Value,
UCase(Fields!Calc_CustPartOpts.Value) = "S", Fields!XPartNum.Value,
UCase(Fields!Calc_CustPartOpts.Value) = "O", "",
UCase(Fields!Calc_CustPartOpts.Value) = "N", "",
True, ""
)
)
)
)
And this is the report using SSRS.
enter image description here
First two items are empty, I can't sort out the cause of this issue, so I thought of using the formula from CR.
Appreciate all the help :)

Answer 1

The logic between the two versions is different, this answer should replicate your CR expression.
This first version simply modifies your structure and changes the check for an empty string in the first line to check for length zero, this will account for NULL/Nothing values too, without the need for two checks.
=IIF(LEN(Fields!XPartNum.Value) = 0, "",
(Switch(UCase(Fields!Calc_CustPartOpts.Value) = "M", Fields!XPartNum.Value,
UCase(Fields!Calc_CustPartOpts.Value) = "S", Fields!PartNum.Value,
UCase(Fields!Calc_CustPartOpts.Value) = "O", Fields!XPartNum.Value,
UCase(Fields!Calc_CustPartOpts.Value) = "N", Fields!PartNum.Value,
True, Fields!PartNum.Value
)
)
)
This version uses SWITCH only to simplify things.
=SWITCH(
LEN(Fields!XPartNum.Value) = 0, "",
UCase(Fields!Calc_CustPartOpts.Value) = "M" OR UCase(Fields!Calc_CustPartOpts.Value) = "O", Fields!XPartNum.Value,
True, Fields!PartNum.Value
)