restsharp/RestSharp · error · ArgumentException

Binary data format needs a byte array as value

Error message

Binary data format needs a byte array as value

What it means

Thrown by the BodyParameter constructor when DataFormat.Binary is specified but the value is not a byte[]. Binary bodies must be raw bytes; any other type is incompatible with the binary format.

Source

Thrown at src/RestSharp/Parameters/BodyParameter.cs:22

// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// 
// http://www.apache.org/licenses/LICENSE-2.0
// 
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// 

namespace RestSharp;

public record BodyParameter : Parameter {
    public BodyParameter(string? name, object value, ContentType contentType, DataFormat dataFormat = DataFormat.None)
        : base(name, Ensure.NotNull(value, nameof(value)), ParameterType.RequestBody, false) {
        if (dataFormat == DataFormat.Binary && value is not byte[]) {
            throw new ArgumentException("Binary data format needs a byte array as value");
        }

        ContentType = contentType;
        DataFormat  = dataFormat;
    }

    public BodyParameter(object value, ContentType contentType, DataFormat dataFormat = DataFormat.None)
        : this("", value, contentType, dataFormat) { }

    /// <summary>
    /// Body parameter data type
    /// </summary>
    public DataFormat DataFormat { get; init; } = DataFormat.None;

    /// <summary>
    /// Custom content encoding
    /// </summary>
    public string? ContentEncoding { get; init; }

View on GitHub (pinned to 6a50821692)

Solutions

1. Convert the value to byte[] before adding it as a binary body (e.g. File.ReadAllBytes or stream.ToArray()).
2. Use request.AddBytesBody(bytes, contentType) which correctly accepts a byte[].
3. For strings, encode to bytes first: Encoding.UTF8.GetBytes(str).

Example fix

// before
request.AddBody(fileBase64String, DataFormat.Binary);

// after
byte[] bytes = Convert.FromBase64String(fileBase64String);
request.AddBytesBody(bytes, ContentType.Binary);

if (dataFormat == DataFormat.Binary && value is not byte[]) throw new ArgumentException("Binary body requires byte[]");

static bool IsBinaryCompatible(object? value) => value is byte[];

try { request.AddBody(value, DataFormat.Binary); } catch (ArgumentException ex) when (ex.Message.Contains("byte array")) { /* convert to byte[] first */ }

When it happens

Trigger: Calling request.AddBody(value, DataFormat.Binary) or constructing new BodyParameter(...) with DataFormat.Binary and a value that is a string, stream, or object instead of byte[].

Common situations: Passing a base64 string instead of decoded bytes; passing a Stream instead of its buffered contents; assuming AddStringBody with binary flag works; serializing an object and labeling it Binary.

Related errors

• Cannot set request body using default parameters. Use Reques
• Request resource doesn't contain a valid scheme for an empty
• File not found
• Invalid character found in header {type}: {value}
• The specified value is not a valid Host header string.

AI-assisted analysis of restsharp/RestSharp@6a50821692 (2026-08-13). Data as JSON: /api/errors/d30b42cb982e2a1a. Report an issue: GitHub.