.cs

static public class CustomTypeMapper
{
    static private readonly Dictionary<Type, Func<OpenApiSchema>> _typeToOpenApiSchema = new()
    {
        [typeof(bool)] = () => new OpenApiSchema { Type = "boolean" },
        [typeof(byte)] = () => new OpenApiSchema { Type = "string", Format = "byte" },
        [typeof(int)] = () => new OpenApiSchema { Type = "integer", Format = "int32" },
        [typeof(uint)] = () => new OpenApiSchema { Type = "integer", Format = "int32" },
        [typeof(long)] = () => new OpenApiSchema { Type = "integer", Format = "int64" },
        [typeof(ulong)] = () => new OpenApiSchema { Type = "integer", Format = "int64" },
        [typeof(float)] = () => new OpenApiSchema { Type = "number", Format = "float" },
        [typeof(double)] = () => new OpenApiSchema { Type = "number", Format = "double" },
        [typeof(decimal)] = () => new OpenApiSchema { Type = "number", Format = "double" },
        [typeof(LocalDateTime)] = () => new OpenApiSchema{Type = "string", Format = "date-time"},
        [typeof(DateTime)] = () => new OpenApiSchema { Type = "string", Format = "date-time" },
        [typeof(DateTimeOffset)] = () => new OpenApiSchema { Type = "string", Format = "date-time" },
        [typeof(LocalDateTimeInterval)] = () => new OpenApiSchema { Type = "string",  Example = new OpenApiString("2023-06-23T00:00:00/2023-06-30T00:00:00") },
        [typeof(Guid)] = () => new OpenApiSchema { Type = "string", Format = "uuid" },
        [typeof(char)] = () => new OpenApiSchema { Type = "string" },
        [typeof(Uri)] = () => new OpenApiSchema { Type = "string", Format = "uri" },
        [typeof(string)] = () => new OpenApiSchema { Type = "string" },
        [typeof(object)] = () => new OpenApiSchema { Type = "object" },
        [typeof(JObject)] = () => new OpenApiSchema { Type = "object" },
        [typeof(TimeSpan)] = () => new OpenApiSchema {Type = "string", Example = new OpenApiString( "00:00:00" )},
        [typeof(Version)] = () => new OpenApiSchema{Type = "string", Example = new OpenApiString( "1.0" )},
        [typeof(IPAddress)] = () => new OpenApiSchema{Type = "string", Example = new OpenApiString( "198.51.100.1" )},
        [typeof(IdentifierCode)] = () => new OpenApiSchema{ 
                Type = "array", 
                Items = new OpenApiSchema
                {
                    Type = "string", 
                    Format = "byte" 
                }},
        [typeof(DateTimeOffsetInterval)] = () => new OpenApiSchema { Type = "string", Example = new OpenApiString("2023-06-23T00:00:00/2023-06-30T00:00:00")},
            
        // Nullable types
        [typeof(bool?)] = () => new OpenApiSchema { Type = "boolean", Nullable = true },
        [typeof(byte?)] = () => new OpenApiSchema { Type = "string", Format = "byte", Nullable = true },
        [typeof(int?)] = () => new OpenApiSchema { Type = "integer", Format = "int32", Nullable = true },
        [typeof(uint?)] = () => new OpenApiSchema { Type = "integer", Format = "int32", Nullable = true },
        [typeof(long?)] = () => new OpenApiSchema { Type = "integer", Format = "int64", Nullable = true },
        [typeof(ulong?)] = () => new OpenApiSchema { Type = "integer", Format = "int64", Nullable = true },
        [typeof(float?)] = () => new OpenApiSchema { Type = "number", Format = "float", Nullable = true },
        [typeof(double?)] = () => new OpenApiSchema { Type = "number", Format = "double", Nullable = true },
        [typeof(decimal?)] = () => new OpenApiSchema { Type = "number", Format = "double", Nullable = true },
        [typeof(DateTime?)] = () => new OpenApiSchema { Type = "string", Format = "date-time", Nullable = true },
        [typeof(DateTimeOffset?)] = () => new OpenApiSchema { Type = "string", Format = "date-time", Nullable = true },
        [typeof(LocalDateTime?)] = () => new OpenApiSchema{Type = "string", Format = "date-time", Nullable = true},
        [typeof(Guid?)] = () => new OpenApiSchema { Type = "string", Format = "uuid", Nullable = true },
        [typeof(char?)] = () => new OpenApiSchema { Type = "string", Nullable = true },
        [typeof(TimeSpan?)] = () => new OpenApiSchema {Type = "string", Example = new OpenApiString( "00:00:00" ), Nullable = true},
    };

    static public OpenApiSchema MapTypeToOpenApiType(
        this Type type,
        OpenApiSchema openApiSchema)
    {
        if (_typeToOpenApiSchema.TryGetValue(type, out var result)) 
            return result();

        openApiSchema.Type = "object";
        
        return openApiSchema;
    }

}