With the growing attention on the distribution activity, the chances grow of implementing distribution automation systems, supported by the Distribution Management Systems (DMS). A software tool central to a DMS is the State Estimation (SE) application, that allows to obtain the best estimate of the current operating condition starting from a set of redundant measurements. The papers presented in literature aim fundamentally at enhancing the computational characteristics of the SE methods. But, with the increasing capabilities of the computing systems, the development of the DMS tools can be approached from the different perspective of flexibility, expansibility, and maintainability. In the paper, a modeling of the distribution systems with computational methods that solve the SE problem is presented, developed with an object–oriented approach. A software tool implemented in an open software CAD environment is illustrated, and numerical results on a test system are presented.