Tag - OleAutomation

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Enhanced and fixed late-binding of variants for Delphi XE2 and up

For several units of our framework, we allow late-binding of data values, using a variant and direct named access to properties:
- In SynCommons, we defined our TDocVariant custom variant type, able to store any JSON/BSON document-based content;
- In SynBigTable, we use the TSynTableVariantType custom variant type, as defined in SynCommons;
- In SynDB, we defined a TSQLDBRowVariantType, ready to access any column of a RDBMS data result set row;
- In mORMot, we allow access to TSQLTableRowVariantType column values.

It's a very convenient way of accessing result rows values. Code is still very readable, and safe at the same time.

For instance, we can write:

var V: variant;
  TDocVariant.New(V); // or slightly slower V := TDocVariant.New;
  V.name := 'John';
  V.year := 1972;
  // now V contains {"name":"john","year":1982}

This is just another implementation of KISS design in our framework.

Since Delphi XE2, some modifications were introduced to the official DispInvoke() RTL implementation:

  1. A new varUStrArg kind of parameter has been defined, which will allow to transmit UnicodeString property values;
  2. All text property values would be transmitted as BSTR / WideString / varOleStr variants to the invoked variant type;
  3. All textual property names were normalized to be in UPPERCASE.

Those modifications are worth considering...
And we may have discovered two regressions: one about speed, and the other about an unexpected logic bug...

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Faster variant late binding

For both our SynOleDB and SynBigTable units, we allow late-binding of data row values, using a variant and direct named access of properties. Thanks to this unique feature (as far as I know in the Delphi database world),

This allows clear and valid code as such:

var Customer: Variant;
  with Props.Execute(
    'select * from Sales.Customer where AccountNumber like ?',
    ['AW000001%'],@Customer) do
    while Step do

In practice, this code is slower than using a standard property based access, like this:

    while Step do

But the first version, using late-binding of column name, just sounds more natural.

Of course, since it's late-binding, we are not able to let the compiler check at compile time for the column name. If the column name in the source code is wrong, an error will be triggered at runtime only. But it would not be an issue, since it would be the same for the SQL code inserted: it's only executed at runtime (this is one of the benefits of using an ORM, by the way: the ORM will generate correct SQL code for you...).

The default VCL implementation of this late-binding was a bit slow for our purpose.
Since it has to deal with Ole Automation, and because it's fun, we hacked the VCL to provide a lighter and faster version for our custom variant types.

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