> ## Documentation Index
> Fetch the complete documentation index at: https://docs.speckle.systems/llms.txt
> Use this file to discover all available pages before exploring further.
# Find objects by property
> Traverse, filter, and index Speckle model data — walls, doors, rooms, and QA checks in .NET
## Goal
Find walls, doors, rooms, and other elements in a received model graph — and build indexes for QA checks, schedules, and quantity reports.
## What you will build
Queries that count elements by category, check required properties, find duplicate `applicationId` values, and group quantities by level.
## When to use this
After `Receive2` on connector-published data — before CSV export, version comparison, or sending analysis results back. Read [BIM data patterns](/developers/sdks/dotnet/guides/bim-data-patterns) first if property names are unfamiliar.
## Recommended approach
**The recommended approach for most filtering tasks is `Flatten()`** on the root object, then filter on `DataObject.properties`. Use `GraphTraversal` with `DefaultTraversal` only when you need parent context or connector-style walk rules.
## Prerequisites
* [Work with model objects](/developers/sdks/dotnet/concepts/objects-and-traversal)
* [Build your first model analysis tool](/developers/sdks/dotnet/guides/build-your-first-model-analysis-tool) — load a model first
On `DataObject` instances, semantic fields live in `properties`: `dataObject.properties.GetValueOrDefault("category")`. Connector-produced BIM data often uses **proxy collections** — see [Working with Proxies](/developers/sdks/dotnet/guides/working-with-proxies).
## Complete example
```csharp Complete example lines icon="https://mintcdn.com/speckle/VtRPWzmN-ULoLfIv/images/developers/sdks/csharp.svg?fit=max&auto=format&n=VtRPWzmN-ULoLfIv&q=85&s=f2970e3b8bd4a2c7899b46211a42f251" expandable theme={null}
using Speckle.Objects.Data;
using Speckle.Sdk.Models;
using Speckle.Sdk.Models.Extensions;
static string? Category(Base obj) =>
obj is DataObject d
? d.properties.GetValueOrDefault("category")?.ToString()
: obj["category"] as string;
// Count walls, doors, rooms
var walls = root.Flatten().Where(o => Category(o) == "Walls").ToList();
var doors = root.Flatten().Where(o => Category(o)?.Contains("Door") == true).ToList();
var rooms = root.Flatten().Where(o => Category(o)?.Contains("Room") == true).ToList();
Console.WriteLine($"Walls: {walls.Count}, Doors: {doors.Count}, Rooms: {rooms.Count}");
// Quantities by category
var byCategory = root.Flatten()
.GroupBy(o => Category(o) ?? "Unknown")
.ToDictionary(g => g.Key, g => g.Count());
// Quantities by level
var byLevel = root.Flatten()
.OfType()
.GroupBy(d => d.properties.GetValueOrDefault("level")?.ToString() ?? "Unknown")
.ToDictionary(g => g.Key, g => g.Count());
// Required property check
var required = new[] { "category", "level" };
var missing = root.Flatten()
.OfType()
.Where(d => required.Any(k => !d.properties.ContainsKey(k)))
.ToList();
// Duplicate applicationId check
var duplicateIds = root.Flatten()
.Where(o => o.applicationId is not null)
.GroupBy(o => o.applicationId!)
.Where(g => g.Count() > 1)
.Select(g => g.Key)
.ToList();
```
## How it works
### Flatten vs GraphTraversal
`Flatten` returns every `Base` in the graph — the right default for filtering and indexing:
```csharp Example lines icon="https://mintcdn.com/speckle/VtRPWzmN-ULoLfIv/images/developers/sdks/csharp.svg?fit=max&auto=format&n=VtRPWzmN-ULoLfIv&q=85&s=f2970e3b8bd4a2c7899b46211a42f251" theme={null}
foreach (var obj in root.Flatten())
Console.WriteLine(obj.TryGetName() ?? "unnamed");
```
`GraphTraversal` adds **context** (parent, property name) and custom walk rules:
```csharp Example lines icon="https://mintcdn.com/speckle/VtRPWzmN-ULoLfIv/images/developers/sdks/csharp.svg?fit=max&auto=format&n=VtRPWzmN-ULoLfIv&q=85&s=f2970e3b8bd4a2c7899b46211a42f251" theme={null}
using Speckle.Sdk.Models.GraphTraversal;
var traversal = DefaultTraversal.CreateTraversalFunc();
foreach (TraversalContext ctx in traversal.Traverse(root))
{
var obj = ctx.Current;
var parentName = ctx.Parent?.Current.TryGetName() ?? "root";
Console.WriteLine($"{obj.TryGetName()} (from {parentName}.{ctx.PropName})");
}
```
`DefaultTraversal.CreateTraversalFunc()` is what Rhino, Grasshopper, Revit, AutoCAD, Tekla, and Navisworks use for receive. Prefer it unless you need to walk different properties.
### Building indexes for repeated lookups
Each `Flatten()` is O(n). Build a dictionary once for multiple lookups:
```csharp Example lines icon="https://mintcdn.com/speckle/VtRPWzmN-ULoLfIv/images/developers/sdks/csharp.svg?fit=max&auto=format&n=VtRPWzmN-ULoLfIv&q=85&s=f2970e3b8bd4a2c7899b46211a42f251" theme={null}
var byCategory = root.Flatten()
.GroupBy(Category)
.ToDictionary(g => g.Key ?? "Unknown", g => g.ToList());
var walls = byCategory.GetValueOrDefault("Walls", new());
```
An `applicationId` index resolves [proxies](/developers/sdks/dotnet/guides/working-with-proxies):
```csharp Example lines icon="https://mintcdn.com/speckle/VtRPWzmN-ULoLfIv/images/developers/sdks/csharp.svg?fit=max&auto=format&n=VtRPWzmN-ULoLfIv&q=85&s=f2970e3b8bd4a2c7899b46211a42f251" theme={null}
var byAppId = root.Flatten()
.Where(o => o.applicationId is not null)
.ToDictionary(o => o.applicationId!);
```
## Common mistakes
| Mistake | Fix |
| ----------------------------------------------- | ------------------------------------------------------------------------------------------ |
| Hand-written recursion on large graphs | Use `Flatten` or `GraphTraversal` — they guard against cycles |
| Rebuilding an index inside a loop | Build the dictionary once outside your query loop |
| Reading `obj["category"]` on all connector data | Use `DataObject.properties` |
| `new GraphTraversal()` with no rules | Always pass at least one rule, or use `DefaultTraversal` |
| Duplicate results from `displayValue` | Exclude `displayValue` in custom traversal rules when extracting BIM properties separately |
## Next steps
Door and room schedules
Resolve levels and groups
Diff between publishes