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Gets or sets the degree of transparency of the specified line as a value from 0.0 (opaque) through 1.0 (transparent).
double Transparency { get; set; }
Property Transparency As Double
IShape connector = worksheet.Shapes.AddConnector(ConnectorType.Straight, 10, 10, 200, 100);
ILineFormat line = connector.Line;
line.Transparency = 0.25;
double transparency = line.Transparency;