CASE WHEN for WHERE clause instead of IF

I am trying to replace IF with CASE, but keep getting syntax error.
Is there way to do something like:
-- SET @ParamSearchType = @SearchCriteria.value('(/params/searchtype)[1]', 'varchar(50)')
-- CASE @ParamSearchType
--   WHEN 'name'  THEN
--       SET @ParamValue = @SearchCriteria.value('(/params/email)[1]', 'varchar(100)')
--       SET @SQLStr  = @SQLStr + ' WHERE LastName = ''' + @ParamValue + ''''
.........
.........

My xml looks like this:


 <params>
   <searchtype>name</searchtype>
   <firstname>Bob</firstname>
   <lastname>Smith</lastname>
 </params>  
 
                                               OR

    <params>
      <searchtype>memberaccountid</searchtype>
      <memberaccountid>100010</memberaccountid>
   </params>


instead of technique with IF statements (btw it works fine the way I need):

Thank you in advance.

@SearchCriteria   xml                  
AS
BEGIN
 
  DECLARE @SQLStr varchar(8000)
  DECLARE @ParamSearchType varchar(50)
  DECLARE @ParamValue varchar(50)
  DECLARE @ParamValueFN varchar(50)
  DECLARE @ParamValueLN varchar(50)
 
  SET @SQLStr  = 'SELECT FirstName, LastName, MemberAccountID, Email.....'
 
 SET @SQLStr  = @SQLStr + ' FROM table'
 
 --Get SearchType value from searchtype parameter
 SET @ParamSearchType = @SearchCriteria.value('(/params/searchtype)[1]', 'varchar(50)')
   
  IF @ParamSearchType = 'name' 
  BEGIN 
        SET @ParamValueLN = @SearchCriteria.value('(/params/lastname)[1]', 'varchar(50)')
        --check if first name is provided
        IF @SearchCriteria.exist('/params/firstname') = 1
        BEGIN 
           SET @ParamValueFN = @SearchCriteria.value('(/params/firstname)[1]', 'varchar(50)')
           SET @SQLStr  = @SQLStr + '  WHERE FirstName = ''' + @ParamValueFN + ''' AND LastName = ''' + @ParamValueLN + ''' '
        END
        ELSE
           SET @SQLStr  = @SQLStr + ' WHERE LastName = ''' + @ParamValueLN + ''''   
  END  
 
  IF @ParamSearchType = 'memberaccountid' 
     BEGIN
       SET @ParamValue = @SearchCriteria.value('(/params/memberaccountid)[1]', 'varchar(50)')
       SET @SQLStr  = @SQLStr + ' WHERE MemberAccountID = ''' + @ParamValue + '''' 
     END
 
  EXEC (@SQLStr)
 
 
 
My xml looks like this:
 
 
 <params>
   <searchtype>name</searchtype>
   <firstname>Bob</firstname>
   <lastname>Smith</lastname>
 </params>   
  
                                               OR
 
    <params>
      <searchtype>memberaccountid</searchtype>
      <memberaccountid>100010</memberaccountid>
   </params>

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