SymbolTable.md

SymbolTable Class

Overview

SymbolTable is the abstract base class for all symbol tables in the AutoCAD database, such as BlockTable, LayerTable, LinetypeTable, etc. It acts as a specialized dictionary that maps string names (keys) to ObjectIds of SymbolTableRecords.

Namespace

Autodesk.AutoCAD.DatabaseServices

Inheritance Hierarchy

System.Object
  └─ RXObject
      └─ DBObject
          └─ SymbolTable
              ├─ BlockTable
              ├─ LayerTable
              ├─ LinetypeTable
              ├─ TextStyleTable
              ├─ DimStyleTable
              ├─ RegAppTable
              ├─ UcsTable
              ├─ ViewTable
              └─ ViewportTable

Key Properties

Property	Type	Description
this[string]	ObjectId	Indexer to get record ID by name.
Count	int	Number of records in the table.

Key Methods

Method	Return Type	Description
Has(string)	bool	Checks if a record with the given name exists.
Has(ObjectId)	bool	Checks if a record with the given ID exists in this table.
Add(SymbolTableRecord)	ObjectId	Adds a new record to the table.
GetEnumerator()	SymbolTableEnumerator	Returns enumerator for ObjectIds.

Code Examples

Example 1: Checking for Record Existence

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    // Open a derived table (e.g., LayerTable) as SymbolTable
    SymbolTable table = tr.GetObject(db.LayerTableId, OpenMode.ForRead) as SymbolTable;
    
    if (table.Has("MyLayer"))
    {
        // Layer exists
    }
    
    tr.Commit();
}

Example 2: Adding a New Record

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    // 1. Open Table for Write
    SymbolTable table = tr.GetObject(db.LayerTableId, OpenMode.ForWrite) as SymbolTable;
    
    // 2. Create Record (Derived type)
    LayerTableRecord newRec = new LayerTableRecord();
    newRec.Name = "NewLayer";
    
    // 3. Add to Table
    // Add() returns the new ObjectId
    ObjectId newId = table.Add(newRec);
    
    // 4. Add to Transaction
    tr.AddNewlyCreatedDBObject(newRec, true);
    
    tr.Commit();
}

Example 3: Iterating a SymbolTable

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    SymbolTable table = tr.GetObject(db.LayerTableId, OpenMode.ForRead) as SymbolTable;
    
    // Enumerate ObjectIds
    foreach (ObjectId id in table)
    {
        // Open each record
        SymbolTableRecord rec = tr.GetObject(id, OpenMode.ForRead) as SymbolTableRecord;
        Application.DocumentManager.MdiActiveDocument.Editor.WriteMessage($"\nFound: {rec.Name}");
    }
    
    tr.Commit();
}

Example 4: Using Indexer

ObjectId id = table["MyLayer"];
// Note: Throws KeyNotFoundException if "MyLayer" doesn't exist.
// Use Has() first if unsure.

Example 5: Case Sensitivity

// Symbol table keys are generally case-insensitive in AutoCAD
if (table.Has("mylayer") && table.Has("MYLAYER"))
{
    // These refer to the same record
}

Example 6: Removing Records

// SymbolTable does NOT have a Remove() method directly.
// You must open the Record itself and call Erase().
using (Transaction tr = db.TransactionManager.StartTransaction())
{
    SymbolTable table = tr.GetObject(db.LayerTableId, OpenMode.ForRead) as SymbolTable;
    
    if (table.Has("LayerToDelete"))
    {
        ObjectId id = table["LayerToDelete"];
        DBObject rec = tr.GetObject(id, OpenMode.ForWrite);
        rec.Erase(); // This removes it from the table
    }
    
    tr.Commit();
}

Example 7: LINQ Query on Table

// Since SymbolTable implements IEnumerable, use Cast<ObjectId>()
var allIds = table.Cast<ObjectId>().ToList();

Example 8: Checking Ownership

// Any record added to this table will have this table as its OwnerID
if (record.OwnerId == table.ObjectId)
{
    // Record belongs to this table
}

Best Practices

1. Always use Transactions: Use AddNewlyCreatedDBObject immediately after table.Add().
2. Check Has(): Checks are cheap; exceptions from invalid indexers are expensive.
3. Erase vs Remove: Remember there is no Remove() method on the table. You Erase() the record.
4. Base Class Usage: Use SymbolTable reference when writing generic code that handles any table (Layers, Linetypes, etc.) identically.

Related Objects

• SymbolTableRecord - The items contained in the table.
• BlockTable - Specific implementation.
• LayerTable - Specific implementation.

References

• Autodesk SymbolTable Reference