E2064 Left side cannot be assigned to (Object Pascal)

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This error message is given when you try to modify a read-only object such as a constant, a constant parameter, the return value of function, read-only properties, or fields of read-only properties.

There are two ways you can solve this kind of problem:

  • Change the definition of whatever you are assigning to, so the assignment becomes legal.
  • Eliminate the assignment altogether.


Examples

program Produce;

const
  c = 1;

procedure p(const s: string);
begin
  s := 'changed';            (*<-- Error message here*)
end;

function f: PChar;
begin
  f := 'Hello';              (*This is fine - we are setting the return value*)
end;

begin
  c := 2;                    (*<-- Error message here*)
  f := 'h';                  (*<-- Error message here*)
end.

The previous example assigns to a constant parameter, to a constant, and to the result of a function call. All of these are illegal. The following example illustrates how to fix these problems.


program Solve;

var
  c : Integer = 1;           (*Use an initialized variable*)

procedure p(var s: string);
begin
  s := 'changed';            (*Use variable parameter*)
end;

function f: PChar;
begin
  f := 'Hello';              (*This is fine - we are setting the return value*)
end;

begin
  c := 2;
  f^ := 'h';                 (*This compiles, but will crash at run time*)
end.

In Object Pascal 2010 and above, E2064 is also emitted by the compiler whenever you try to assign a value to a member of a record exposed by a read-only property. Consider the following record type:

TCustomer = record
  Age: Integer;
  Name: String;
end;

that is exposed by a read-only property in a class:

TExposing = class
...
  CurrentCustomer: TCustomer read SomeInternalField;
...
end;

Assigning a value to either the CurrentCustomer property or to a member of the record exposed by the CurrentCustomer property results in the compiler emitting this E2064 error.