What is the difference between NULL and "" ?

[macpee]

Can someone tell the difference between

NULL

and

""

Thank you in advance.

[Keith Watford]

NULL is basically no value as far as I understand when a string variable is assigned.

Note

string a=NULL;
Print(a=="");

‌Will not return true

whereas

string a=NULL;
Print(a==NULL);

‌and

string a="";
Print(a=="");

‌will both return true

[Ex Ovo Omnia]

I would recommend to avoid NULL in MQL completely unless you have tested behaviour for the particular datatype. It is implemented inconsistently, for string it means an uninitialised string, while for integer it means either an uninitialised integer or zero, for object pointers it means uninitialised or void objects however not the destroyed pointers.

[William Roeder]

I agree, (except for pointers where you specifically check for NULL,) don't use it.

You can only use NULL in calls where the documentation specifically mentions it, and those are mostly a substitution for _Symbol.‌