Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

Assignment Compatibility
Assignment-compatibility is not a symmetric relation. An expression of type T2 can be assigned to a variable of type
T1 if the value of the expression falls in the range of T1 and at least one of the following conditions is satisfied.
T1 and T2 are of the same type, and it is not a file type or structured type that contains a file type at any level.
T1 and T2 are compatible ordinal types.
T1 and T2 are both real types.
T1 is a real type and T2 is an integer type.
T1 is PChar, PWideChar or any string type and the expression is a string constant.
T1 and T2 are both string types.
T1 is a string type and T2 is a Char or packed-string type.
T1 is a long string and T2 is PChar or PWideChar.
T1 and T2 are compatible packed-string types.
T1 and T2 are compatible set types.
T1 and T2 are compatible pointer types.
T1 and T2 are both class, class-reference, or interface types and T2 is a derived from T1.
T1 is an interface type and T2 is a class type that implements T1.
T1 is PChar or PWideChar and T2 is a zero-based character array of the form 
array[0..n] of Char
 (when
T1 is PChar) or of WideChar (when T1 is PWideChar).
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T1 and T2 are compatible procedural types. (A function or procedure identifier is treated, in certain assignment
statements, as an expression of a procedural type.)
T1 is Variant and T2 is an integer, real, string, character, Boolean, interface type or OleVariant type.
T1 is an OleVariant and T2 is an integer, real, string, character, Boolean, interface, or Variant type.
T1 is an integer, real, string, character, or Boolean type and T2 is Variant or OleVariant.
T1 is the 
IUnknown
 or 
IDispatch
 interface type and T2 is Variant or OleVariant. (The variant's type code
must be 
varEmpty
, 
varUnknown
, or 
varDispatch
 if T1 is 
IUnknown
, and 
varEmpty
 or 
varDispatch
 if
T1 is 
IDispatch
.)
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