syntax error at or near "=@" when using parameters

I'm trying to update/insert data to postgresql depending whether given id is already preset in the table.

This works for update

UPDATE table SET 
   column=@Column
WHERE id=@Id;

And this works also (hardcoded values)

DO 
$$
BEGIN
IF EXISTS (SELECT * FROM table WHERE id=123) THEN
 UPDATE table SET 
    column=123
 WHERE id=123;
ELSE
 INSERT INTO table(id,column) VALUES (123,123);
END IF;
END
$$

But as soon as I try to give values as parameters, it breaks giving an error syntax error at or near "=@"

DO 
$$
BEGIN
IF EXISTS (SELECT * FROM table WHERE id=@Id) THEN
 UPDATE table SET 
    column=@Column
 WHERE id=@Id;
ELSE
 INSERT INTO table(id,column) VALUES (@Id,@Column);
END IF;
END
$$

To isolate the problem leaving just one parameter like this

DO 
$$
BEGIN
IF EXISTS (SELECT * FROM table WHERE id=@Id) THEN
 UPDATE table SET 
    column=123
 WHERE id=123;
ELSE
 INSERT INTO table(id,column) VALUES (123,123);
END IF;
END
$$

Gives error is Npgsql.PostgresException: '42703: column "id" does not exist'. So is there some kind of magic I have to do with the parameters?

Solution

Npgsql does not support parameters within dollar-quoted string literals ($$).

However, you seem to be implementing upsert (update or insert) - PostgreSQL supports that natively with INSERT ... ON CONFLICT syntax. See this tutorial or other documentation on this feature, which should obviate the complexity of what you're trying to do.