Field

Fields are discovered automatically via Init() and then configured before Load(). The engine is schema-less — you never declare fields up front.

engine.Init(fstream);

engine.GetField("name")!.Searchable = true;
engine.GetField("name")!.Weight = 2.0f;
engine.GetField("description")!.Searchable = true;
engine.GetField("category")!.Filterable = true;
engine.GetField("category")!.Facetable = true;
engine.GetField("price")!.Filterable = true;
engine.GetField("price")!.Sortable = true;

Configuration Properties

Read-Only Properties

Setting PreloadFilters = true automatically sets Filterable = true on the same field.

Fields With No Type

A field that was null or absent in every document Init() read has no type. Filterable and Sortable are the only two roles that need one, to choose between a numeric and a categorical index and to pick a comparator, so both are refused on such a field: TrySetFieldConfiguration returns false with the reason, and the setters on Field throw ArgumentException. Clearing either is always allowed.

Searchable and Facetable do not read the type and stay available. Setting Searchable on an empty field prepares it: a record inserted later that does carry a value is indexed and found.

Changing Configuration After Load

A change needs one of three things, and you can ask which before applying it:

var proposed = new[] { new FieldProxy { FieldName = "category", Facetable = true } };

if (engine.DocumentFields.RequiresReload(proposed))
{
    // Searchable, WordIndexing, Embeddable, PreloadFilters, BM25k1 and HighResolution.
    // Only Load consumes these. Applied in place they return the engine to Created,
    // so searches fail with a reason instead of answering as if the field were empty.
}
else if (engine.DocumentFields.RequiresReindex(proposed))
{
    engine.TrySetFieldConfiguration(proposed, out string error);
    engine.Index();
}
else
{
    engine.TrySetFieldConfiguration(proposed, out string error);   // takes effect at query time
}

RequiresReindex covers a field getting its first role. A document keeps values only for the fields in use when it was loaded, so such a field has none until Index() reads them back from the stored JSON. A second role on a field that already had one takes effect at query time.

To apply a reload-only change without taking the engine out of service, build the new index beside the live one with CreateInMemoryClone(monitor, fieldOverrides) and swap it in.

FieldProxy

FieldProxy is used by GetFieldConfiguration() (returns FieldProxy[]) and TrySetFieldConfiguration(FieldProxy[], out string error). All boolean and float properties are nullable — null on a set call means "leave unchanged"; every property is populated on a get call.

// Read the full configuration
FieldProxy[] config = engine.GetFieldConfiguration();

// Apply changes — only non-null values are written
engine.TrySetFieldConfiguration([
    new FieldProxy { FieldName = "title",       Searchable = true, Weight = 2.0f },
    new FieldProxy { FieldName = "description", Searchable = true, Weight = 1.0f },
    new FieldProxy { FieldName = "category",    Filterable = true, Facetable = true },
    new FieldProxy { FieldName = "price",       Filterable = true, Sortable = true },
], out string error);

Scoring Modes

Setting BM25k1 differently across fields controls which scoring algorithm the engine uses after Index():

// BM25F mode (default) — uniform k1, single shared index
engine.GetField("title")!.BM25k1 = 1.2f;
engine.GetField("body")!.BM25k1  = 1.2f;   // same → BM25F

// BM25PerField mode — divergent k1, one index per field
engine.GetField("title")!.BM25k1 = 2.0f;
engine.GetField("body")!.BM25k1  = 1.2f;   // different → BM25PerField

Inspect SearchEngine.ScoringMode after Index() to confirm which path was selected.

Field Weights

Weight is a plain float. The practical range is 0.5–3.0. The ratio between fields matters — 2.0 and 1.0 produces the same relative effect as 4.0 and 2.0.

engine.GetField("title")!.Weight    = 3.0f;
engine.GetField("heading")!.Weight  = 2.0f;
engine.GetField("metadata")!.Weight = 1.5f;
engine.GetField("body")!.Weight     = 1.0f;

There is no hard maximum, but values above 5× the lowest weight in your set offer diminishing returns. If one field needs to dominate that strongly, it is usually better to make it the only searchable field.